Oneitis

Catria
Would try and love you, but unable to get over Marth she'd leave you since she doesn't want you to be a replacement.
Wouldn't cheat, but if you do she'd blame herself.

Palla
Wouldn't even try to be in a relationship with you, she'd rebuff all of your advances and never put out if you do get her to date you. Relationship would be devoid of any love from her end (or so it seems). In reality she can't get over Abel and doesn't want you to deal with that baggage.
If you cheat she kicks you out on the spot.

Cordelia/Tiamo
Would love you, but not as much as Chrom. Infidelity is almost a daily case with her, and it only gets worse if future Severa is around. Would cry and kill herself if you cheated on her.

Severa/Selena/Luna
Would be angry and very demanding, but not so secretly be incredibly fearful of other women near you.
Wouldn't cheat on you unless her mom is there, in which case she's fuking every man in a kilometer. Once you found out she'd blame you for it, and berate you for breaking up as a result.
Would never forgive herself for what she did and end up just like her mother with you in place of Chrom.

Katarina/Eine/Reese
Clingy, expects you to have sex and be romantically involved with several other women while you're with her.
Any attempt at staying monogamous will drive her info a panic and she'll hire prostitutes for you.
Would never cheat on you.

Kleine/Clarisse
As clingy as Eine, expects you to cheat but doesn't want to know about it. Wouldn't cheat on you, and is likely to kill you if you try to leave her.

Faye
Doesn't love you, will never love you. Makes it no secret she yearns for Alm and you're just there occasionally cumming inside her. Leaves a lot for days on end, but will never cheat on you unless it's with Alm who isn't interested. You're more likely to fuck Alm's wife Celica than Faye is to cheat on you (which granted isn't much as you're probanly gonna fuck the qeen anyways but the point is made, Faye is faithful 100%) and Faye honestly wouldn't give a shit no matter how unfaithful you are.

Titania
She'd never consider it and tell you to go away. You can't cheat on her as you'd never date her.