saying you have a girlfriend/boyfriend does not always stop advances, many people cheat and some people are poly. Its easily possible he still would have clarified his intention on the roof.
I think the reality is, going on something that could be a date, with someone you have no interest in, ends up awkward like 50% of the time. This is very true irl, so not sure why yall think it wouldn't be in game. River is not presented as a slick guy, hes the type to go on a roof and just spurt out his feelings.
it'd have went like this, "I know you got a girlfriend but, I feel a connection to you... I just need to know, do I have a shot?"
river was gonna go down hard, no matter what, if you weren't interested in him.