What is Star's Mewninite (NSFW)

Mewninite to Star is akin to Kryptonite to Superman. It does differ in intensity and a full reaction. Mewninite isn't physical either. It manifests inside of their True Soulmate, and as such only affects them. For some it comes and goes quite quickly, for others it lasts until a certain point is reached. How about the princess of Mewni?

Star's Mewninite is Marco.

After they became smooch buddies and share their kiss, Star felt weak.
She thought it's just Marco's manliness working on her like it did on Janna, Hope, and Brittney.
With every loving moment they share, she weakens.
Star doesn't say anything to Marco. Leaving him to his suspicions.
This didn't stop the romance or the love.
For every day Star couldn't walk, Marco would carry her. For every time Star was too weak to leave bed, he would stay by and comfort.
They were inseparable
Fast forward a few years
The first Earth/Mewni Wedding
Star and Marco's parents and all their friends came.
Janna was trying to cast every curse on Star she can think of. Some curses were cast out of jealousy, others out of guilt for the previous spells.
Star's at the altar, waiting for Marco and his mother to walk down the aisle. Mewni tradition that the groom didn't mind following.
They go through the ceremony and kiss. A kiss that involved Star getting dipped and an audience going "Wooooooooooooooooo!"
Star faints, everyone assumes it was just the excitement because there's a smile on her face. But the overexposure was the catalyst.
After she awakens. Marco's there with her, holding her hand in a classy hotel room. Some call the penthouse suite
They embrace each with a long hug
If there was anyone there, you'd think there were hearts floating around the two.
No wait there are.
Oh god why are they purple?!
The married two don't notice and just go into a deep kiss. Arms wrapped around each other. Hands on each other's heads.
Marco and Star get even more intimate.
It's about to get lewd-o.
The new groom whips about the patent pending Marco brand D-iaz
The mating ritual is initiated.
On first insertion Star screams with both pleasure and pain.
Her body instantly becomes purple with her wings spread and 4 extra arms sprouted.
Marco's surprised, but not done selling his package.
It's not delivery. It's Marco P.H. Diaz
The continue, with Star just getting more and more purple. Seemingly more and more lost into the transformation. She isn't weaking, at least.
Star's top arms grab the top of the bed. Her bottom arms hold the bed for leverage. Her mid right grabs Marco's head and brings it in for a kiss, while the left grabs his ass.
The misunderstood bad boy exclaims with an "Ooo." Never having had his butt grabbed so fiercely before.
Purple hearts start swirling around the two, like a whirlpool of violet love.
Whether it's fortunate or unfortunate, they both start getting close.
It was their first time afterall. No matter what the Diaz family plan entailed, safety boy avoided it like the plague.
The both reach their climax
Marco's package get's delivered, left his own signature, and is now exhausted but smiling
The beautiful and perfect bride-urfly explodes with purple hearts going everywhere
Star, herself, is heart free and back to normal. Marco wasn't so lucky
He laughs. "I'll be spitting out those for days"
Star looks over to Marco and exclaims
"I'M SO SORRY. THIS NEVER HAPPENED BEFORE"
Marco gives a smile cheesier than any nacho he's ever made
He just hugs Star and says
"That's alright. I'll always love you Star. Even if I need mouthwash to wash this out. Plus, we'll get better at this in the future."
They hold each other, basking in the afterglow. Before long, they fall asleep together.
Smiling.