Alexis figure out who MC was on the plane before banging him? Then we find out she was his first kiss so it's not just some random that's her type.
She did, which actually makes it so problematic, imo -- if it was just a matter of ignorance and random hookup that'd be a different thing altogether.
You can learn from Alexis that she actually resented the MC "stealing" her first kiss. She tells you at some point (once the truth of her knowing MC's identity from the start comes out) that she changed her mind on that only recently, after she's dated him for a while and got to know him better. So it's not like she had any real positive attachment to the guy she decided to bang in the plane toilet, while knowing it's someone her supposed best friend had real fondness for. Quite the opposite if anything.