Why do you morons always ask this question?
If these Elves were real, there would be no choice for you. You wouldn't get to pick one of them; an Elf would pick you, and there'd be nothing you could do about it. Nothing.
Blood Elves and High Elves have at least some mastery of magic, and would just overpower you just like that. Don't even get me started on the actual competent Mages, Priests and Warlocks who can and will enslave you if needed.
Void Elves have their void tendrils and can easily constrict you until you give in or die.
Night Elves are either physically overwhelming, Druids who can use nature to subdue you, or also masters of magic.

Truth is, the moment an Elf has her sights on you, you belong to her. Doesn't matter if it's a tall, strong Night Elf or a lithe Blood Elf. You're bound to be a slave to an Elf, if they were to exist. Girlfriend or not. They'd just get rid of her anyway. Night Elves and Void Elves will just kill them outright, and the Mages will probably polymorph your girl into a sexy pair of panties and wear them while they conquer your heart and mind.

Be glad Elves don't exist, because you'd most likely end up as the unwilling servant of one.

You think it's a fucking joke? You think you'd have a good time being an Elf's literal bitch?
They'd have you do all sorts of shitty servant stuff, like making her breakfast and going down on her whenever she wants. Helping them do their makeup and style their hair, serve them as they bathe and be dragged along as they go shopping for new dresses and lingerie.
You'd have to go with her as she goes about their day, being shown off to her friends and maybe even being made to worship her when your loyalty is questioned by her jealous acquaintances. And everytime a human woman shows up, your Elf mistress will force you to hold her hand and act like a loving couple, just to stick it to the round eared women.
The worst of it all, is that when you're alone together, she'll drop that demeaning and demanding attitude, and instead trick you by showing compassion and love. She'll hold you close, caress your face while looking down at you with those glowing eyes, and let you rest your head on her warm thighs or soft belly. She'll make you believe that, when nobody is looking, she actually loves you, her favorite servant. She would trick you with such acts so much that you'll undoubtedly end up submitting to her entirely, only to find yourself licking her ass and kissing her lower lips like a little slut.

You wouldn't actually be willing to subject yourself to that?

Why would you even do that?
You'd literally ask her to be turned into a pair of soft panties, to feel that strange sensation of being capable only to feel her slide you up her legs, to feel her warm thighs against a small part of your new "body", until you find yourself nudged between her cheeks and pressed against her mound?
You'd want her to wear you like that, to feel the warmth of her beautiful body always pressed against you, to take in the smell and growing wetness of her arousal overwhelming you, becoming part of you?
I mean, surely you don't want to be literally worn by an Elven beauty when she starts making out with your loving girlfriend, able to hear but not see your woman slowly giving in to the seductive ministrations of an Elf, whose warm lips press against your girlfriend's while the Elf's lower lips press against you? To feel as much as hear the Elf growing more and more lustful by your girlfriend's submission, evident by her increasing arousal that makes you nearly pass out - if at all possible now turned into her lingerie.
Come on, such a fate would be disgusting and humiliating. There's no way anyone would want to feel her asscheeks surround part of you, while her lips gently grip the fabric of your new body, all the while the woman you loved is blissfully unaware in her submissive state, and ready to call the Elf her mistress and lover.

Exactly. That's one of the worst things of being a slave to an Elf.
Every day, every single day, she'd have you enter the large bath with her, and make you sit behind her. Instructing you time and time again to lie back a little, so she can rest her back against your chest while she relaxes, and making you massage her ears like some slave,
Having her press her entire body back against yours, her warmth on your chest and her way too perfect ass grinding up against your crotch, while you have to gently but thoroughly tend to her long, pointy ears. To feel them between your fingers as you run them around the length of them, applying pressure just enough to be relaxing, and occasionally kiss them when she demands it.
It's a stupid, annoying and degrading act that she'll subject you to over and over, until you're so good at it that your fingers sliding over her ears is enough to make her sigh in relaxation and appreciation. You'll be reduced to nothing but a silly slave, gently rubbing her ears while unable to ignore the fresh smell of her hair up in your face.

Blood Elves are especially vile. They'll force you not only to make dinner for them, but share in that meal as well. You'll be sitting at a table with your mistress and be made to enjoy her company, to see her smile softly as she appreciates your improved cooking. You'll be constantly looking forward to the end of the meal, where she will thank you for your loyalty with a long, loving kiss before cuddling with you on the sofa.

No way. There was an Anon who thought he could bring his girlfriend into an Elven city, so that if he was hunted by an Elf he could just give her up instead and run.
Now, some Elf has herself a submissive couple instead. Nothing works to avoid such a horrible fate.
If you struggle, she'll only french kiss you more, cuddle with you while bathing and softly hum when you tend to her needs, inevitably overflowing you with unwanted affection for your new mistress.
Whenever you apply her makeup or braid her hair, she will smile at you, wink when your face is inches away from hers, and plant a quick kiss on your nose when she catches you by surprise. She will force you to fall for her, and every night she will wrap her arms and legs around you to hug you tight, to make you feel her warmth, smell her scent and so you will dream of her for certain.

And if you think about submitting to her, so that she will be surprised by your feigned loyalty, think again. If you force yourself to give in to please her so that you may escape her, she will merely have you work even harder. She will sit on your face instead of letting you crawl between her legs, and not let up so you can breathe. She will make you kiss her butt even more often, and your massages will extend to you tending to her entire body - with happy endings mandatory.
Even if you'd start by giving in, there would be no way that you can resist her increased sexuality. Before long, you will find yourself between her thighs multiple times a day, her warm flesh against your face in a recurring ritual, and by then you will have genuinely given in to her. You will yearn for her juices, and work double to be granted a taste of her, and you will no longer do this in an attempt to lower her guard, but to have her release in lust so you can drink from her again.

You'd be doomed, no matter what.

Utterly disgraceful. To be reduced to a servant, to make her a cake that she enjoys so much. There's no doubt about it, she would take the cherry and force a sexual interaction with you. Teasingly drawing it over her body, dipping it lower and lower, and you'd know full well she would have you on your knees beside her, so that she can give you a taste of the cherry mixed with her nectar.
Only after submitting yourself to such a vile act, would she place a finger under your chin and, with a smile, pull you up until your lips are close to hers, close enough for your noses to touch.
She'll kiss you, in ways no human woman can. She'll hug you, and thank you in exaggerated appreciation, making you feel all the more a servant to her basic needs.
And the fact that you made her a cake - she didn't even explicitly ask for it - will yield you an unwanted reward. She'll probably take you out for a "romantic" walk, maybe even meet your family again, where she will express how fond she is to have you in her life.
Not to speak of the night that follows, where she will annoyingly prove her superiority over human women by giving you release several times, some of which caused merely by serving her with your tongue.

You can't, really. Some aren't chosen by an Elf, but the increase in their (combined) population means that soon, every male human will fall prey to an Elf.
And if you run, or hide, you only make yourself more desirable to her. She'll find more joy in taking you, and the adrenaline and thrill of the hunt will no doubt send her into a sexual frenzy.
There were some who ran, but take their fate as a lesson. You will be caught, and she will make fierce love to you for hours - even days if you last that long. Some have died, even, their hearts unable to cope with the love and lust forced upon them, their bodies simply surrendering to their embrace. But more, most really, only find themselves at the Elf mistress' feet, their minds unable to think of anything other than hearing their lover moan in appreciation, to see that warm smile and feel Elven legs wrapped around their heads.
You will become an Elf's lover, there are simply many paths to that fate, but none that stray from it.

Sure. It doesn't sound bad to wake up every day, looking at the face of an Elf who is in pure bliss thanks to having such a loyal servant.
It doesn't sound so bad, to always be so close to an Elf, to see that annoyingly perfect face, with those warm glowing eyes, soft lips, long ears. To know that it is the face of your mistress, your owner, and that you will have to tend to her for the rest of your life.
She will be everything to you, and you will end up living only to hear her appreciate your deeds, to see her lie back with her eyes closed and with a smile on her face. To lie in her embrace while she drifts to sleep, utterly relaxed from your late night massage.

Doesn't sound so bad? It sounds like a horrible fate to me.