MC can play like that, or he can be faithful, from what I read the pregnant GF can't stay faithful. That is the difference between canon forced scenes/relationships and optional ones.
There is no lesbian stuff happening
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, just a kiss that the GF apparently barely rejects and doesn't tell anything to the MC? How would you react if this was your gf/wife,
the mother of your child doing that? And since apparently 'lesbian are hot' messes with people's common sense, how you would react if your wife was getting kissed behind your back and barely rejecting the constant advances of another dude?
Reading some more comments, gotta love the backwards justification that just becasue you might end up fucking both of them in the end, its ok to have the most predatory and cucky setup possible. How throuples/threesomes are setup changes players perception of characters, if there is cheating, sneaking around and disrespecting the MC/player involved, than it's a shit throuple with shitty characters.
One more thing, if what you are insinuating is true, babysitting LIs so they don't cheat might also be the least fun playthrough ever imaginable. If she can cheat at the click of the mouse on the 'wrong' choice, why is she even worth the time and effort for a player that isn't into cuck content?
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Have you noticed how its always the predatory type of setups that are unavoidable in these type of games? The devs never give the option to dropkick the LI(s) doing that shit. You are almost always forced to be ok with it,
the MC likes it, or is too
open-minded and
progressive to make a big deal out of
cheating 
and in the cases where a breakup choice is given it comes way later and the MC never gets to throw this behavior in their face, so the player never gets vindication...