Monster Basher Brutal 3000 for console was his first desire for a romantic moment, so I'm not going to put too much credence to young William's instincts.
Incidentally, doing Mon on the kitchen table? So much classier than a bed. It's almost laughable if the concept wasn't so horrible. I mean, did they push the plastic cup of coke away, the bowls of chips and boxes of pizza, the empty plates? I mean kitchen tables are very used items at parties. Pretty much anywhere else is easier. So not only does he manage to clear some space without killing the mood, no one comes looking for snacks while they're going for it. Sure it's going to be quick, but damn. The whole situation is crazy.
I find this scene really funny, but I can't fault William too much, who would not want it ?
Mona thing... Yeah, is not great but as was said I don't think is the crazier thing in this game, how the hell does Leia goes from anorexic kid mob boss ?
. When talking about first choices, I'm kinda thinking back then William just went for whatever he spied with his eye beginning with horny.

At least we have (crocodile?) tears from him now.
I think this is a very fair assessment, young William seems inconsequential at best and his behavior until now does not seems that he changed much, he does seem genuine in his desires to right the wrongs he did though so at least he knows he was a complete ass.
Im not entirely sure how old they were supposed to be at that point but i was on partys like that since i was 15 (not anymore, im old now), its not really strange here in germany for most people living in cities (beer is legal with 16 anyway and hard alcohol with 18).
And willy very obviously was a rude asshole horny teenager but in his defense he was young and emotionally confused (about his feelings and relationship with his twin sister). We dont really know what happened, he may tried to get over his "unnatural" attraction to Leia and simply realised hes not able to (with post nut clarity lol) or whatever.
I have talked about it before but trying to make sense of the timeline gives me headache, now I am just rolling with it.
Not to defende William, but while what he did was terrible, what Leia did was 10 times worse, she went full psycho on Mon and spent a year (?) calling her cumdump. While William has a excuse of acting on instinct and with out thought, what Leia did was thought through and meticulous, truly evil. I also don't want to make too much assumption over Monica, while he does is the victim in this situation I don't think that a single character in WiaB is "pure" and I think that we will find out that there is more to Monica than just the naive teenage that William did wrong.