edit:
i'm interpreting too much into this dumpster fire...
the whole setup is more suitable for a drama than a romcom IMO, but it isn't one.
and by the time the "why's" get answered there will be nothing left.
"why a drama?" you ask? cause if this was a drama, the drama could revolve around how pathetic and shallow the MC is, and how he gets abused to fulfill the fetish of an honor student(?), who is struggling with stress, or some shit.
the MC questions their relationship, but he likes it anyway, cause girl 1 is "cute".
pathetic and unhealthy if you ask me.
BUT this is a romcom not a drama, so the reason why she approached him will be something along the lines of
"i like you for whatever reason and approached you with the 'do me like your eroge girls' reason, cause i know you like that shit."
@Echonic AFAIK aniki is the male version and aneki is the female version. don't quote me on that.