Yet interestingly enough as soon as they break up Mike starts to reach out and contacts Lena on his own, asking if maybe she'd still like to hook up, something he doesn't do before. Lena comments on it, that "maybe he didn't give up on her". He may be chill about it on the surface, but he's still a guy and he wants to stroke his ego by having a hot girl jump on his dick whenever he requests it. (instead of facing the music she'd rather do it with someone else)
From a more cynical angle, the number of guys in the game is limited, and Lena/Ian relationship requires some guys for Lena to fuck behind Ian's back if that's the player's preference, so the guy she's already fucked and potentially already cheated on Ian with, isn't really going to suddenly turn all nice and meek about it, no longer providing any temptation.