I believe Rinta wants the sexual attention he's getting. He does the moral grandstanding when Fumi pushes him too much, but does nothing to address the issue because he wants things to keep going. Surely if he wanted this to end he would come up with some basic solutions (like, as TheBlackGamer mentioned, recording her in secret to prove his innocence).
In this sense, perhaps Fumi is correct to think of him as a hypocrite. "Sure, you portray yourself as not wanting my advances but you do nothing to stop me." Granted, his arousal in this chapter was warranted and her crusade against liars (and him in particular) still remains a stupid one.
Rinka came from the sticks where he only had like 3 other classmates. They literally don't have trains or other advanced technology of the big cities there. I don't expect this guy to have the presence of mind or the ruthlessness to realize he can also be devious enough to fight fire with fire by recording her. And knowing his naviete and his open book personality, she will likely catch onto any recording attempts he makes since she has a very good read on his mind when he's nervous. He will not be smooth with it, he's too inexperienced.
And even then, the truth is that the first impression of a sexual crime/rumor matters A LOT. Her credibility on campus is that of Queen Bee, literally. Any initial disclosure of evidence from both sides will be seen in two completely different ways. She is way more socially shrewd to know how to push her agenda/evidence to the forefront and poisons others' mind against his innocence.
What she has a pic of him over her on the night she was drunk and he was not, and truth is that women make for much more sympathetic victims than men do. Even if he records her, if she manages to get her own version of events out before him (which she will likely succeed at since she has a much bigger social circle and presence), he will initially suffer a huge social reputation cost, which will likely never be completely recovered even if he presents evidence that acquits him, because the court of public opinion is capricious and usually yields to the more sympathetic narrative. As a woman she has a clear advantage here, especially since she puts on a "pure" persona on campus, whereas he's the inexperienced hungry virgin. No one will believe him over her in the initial fallout.
While he works to recover his reputation using his evidence, in the meantime the things that matter to him will be destroyed, his connection to the childhood friend, his standing in his favorite club, college reputation, etc.
Fighting fire with fire means everything turning into a shitshow he has no guarantee of coming out on top without massive damage of his personal relationships and social reputation. Bro might not go to jail but he will be in social jail for a long time.
As for about him "welcoming her advances" I genuinely have no idea how that point wins out over his helplessness in this dynamic.
That entire scene with her pestering his body while his friend was outside the door was really difficult to read. That has to be the most helpless depiction of assault I've seen so far in this series, and the whole time he cannot even raise his voice or even push her away for fear of giving himself away to an inevitable misunderstanding and smearing of who he is as a person. What the hell do you want him to do at this point? His body is reacting, but that's just because he's a healthy straight man. That's not proof of him being a disgusting person.
If he was "welcoming her advances" he would actually reciprocate. He would actually touch her. Every time he touches her is when she forcibly moves his hands to her body. I stg if the genders were reversed, no one would ever be saying that "she was welcoming it"
Men have a disadvantage in this regard since their physical arousal is much easier to achieve and has a way higher threshold for disgust before it can negatively effect arousal.
For women it's the opposite. They require a much higher standard for safety and mental peace before they can be fully aroused, and they have a much lower threshold for disgust. That's a legitimate, replicable, and documented sex difference in the literature.
So while it may seem like his body is welcoming it, his heart clearly isn't in it, even if he feels temptation. Which imo isn't enough to make him a disgusting person cause which inexperienced college virgin wouldn't feel tempted to jump at the chance to lose the one thing their entire social cohort would mock them for not losing - their virginity (and plus the regular temptation a man feels to touch a woman sexually duh)