She forces herself on him. Then he continues to knowingly facilitate opportunities for her to do so. Dude straight up invites her to his house to study and is surprised when she takes it seriously instead of getting handsy - after she straight up dragged him into the bushes to assault him at their last meeting. That is, until she finds a condom planted there as a prank by the resident crazy bitch featured in this chapter and desperately attempts to convince him to use it with her
I'm not one to engage in victim blaming, but dude has no one to blame but himself for allowing Yuka to continually assert herself after the first time she throws herself ontop of him and kisses him - because it only escalates from there and he makes no real attempt to stop it.
It doesn't help that he laments the fact that she "turned him into a lolicon."
With a gender ratio as bad as 1:5 the social norms we see are just everybody playing along because they're not quite at the point of "Holy fuck, we're going extinct" like in 1:39. But if 3/4 out of 5 girls aren't getting a guy, I really don't see age gaps mattering anymore. Personally, I give the girl props for smelling the BAREST whiff of drama and deciding that she'll do WHATEVER she needs to for him to take her as a serious romantic interest; it's honestly the right mindset to have for a girl in the setting.