Use spoilers.
Not sure... but consider the over all issue with the MC. He fights with the concept of her being some disgusting gangbang whore, this is what he has been shown concerning the college days. This is who he has come to know in that exchange with Mia, this was her past, this is her to be honest at her deepest reveal.
Then consider he just dealt with a conversation with her PUSHING him to bet her, you know... because as she said, she doesn't consider anything about being passed around by a bunch of guys who want to treat her as such.
So, maybe he is angry, angry at that reality which he has been dealing with of her past, angry at the fact that their relationship means so little to her that she would wager herself to be some cum dump whore again without thought or concern to the MC, you know... because this is old hat for her... and so he like the JD goes all in, not accepting, not approving, but simply spitting her own disgusting words out openly for all to hear... "Lacey is a whore, a fucking whore who will become a cum dumpster for all who wish to deposit, this is my wife, the fucking disgusting whore who will do anything you desire, etc..."
That may be "one" reason, you could be right, but honestly who knows... the professor isn't exactly consistent in the characters, but I will say, him being "accepting" or becoming "corrupted" as you say does not fit the character at all at this point. He has shown no real signs of any note that he likes this side of her or approves of it.
If anything, the poker game shows how little Lacey has grown, how she contradicts with her proclaimed healing "between" the MC. Maybe she has come to terms with her acceptance of her path if you want to make that argument, but if you do, it is accepting she is a depraved whore with no bounds or respect for the MC.
If Lacey wanted to show extreme devotion to the MC, she would have said.... "No, do not gamble me, do not risk this... the money means nothing, we can always earn more... I would rather you respect me, love me for what I devote myself to, and that is to you, and you alone"
Now that would be a sacrifice knowing her character, because she could EASILY be the whore for a night like she said, but to refuse, to devote to her husband in the face of the set back it would have financially? That is devotion. Lacey showed none, she risked nothing, she offered nothing. A night of her being a gangbang whore would be a pleasure for her physically, regardless if she said "I don't want to be shared". It would be a respect to the MC, but by her pushing, it showed none... no sacrifice, no growth.
Remember, she knows this will destroy him, but for a little extra cash... she is willing to put him through that, because she says she will nurture him back... you know, because she has been doing such a good job so far of that.
About the subject of whatever the MC is getting corrupted or not, I'd point like I mentioned in my previous comment, the example of Yue, she had been hesitating between the MC & Bradley, but she had clearly chosen the MC, they go on a date, she is basically being all cute over him, having bought a dress to match his outfit and all. And he tell her go for Bradley, now if he had just said that it would mean he is merely trying to stir her away from himself, but at that point he add that they'll figure it out later, what do you think that mean?
It mean he is literally telling her to date Bradley, and if it doesn't do for her he'll take care of her needs, he is literally setting up a Netori relationship for himself, he outright chose the cuck his girl would take and told her he'd take care of her needs later basically, now it might alternatively end up in swinging, or you'll have the choice to reject her and leave her miserable, but the point the MC is clearly setting himself up for potential Netori, and you don't call that him having grown corrupted?
Yea he might not be happy about the notion of sharing Lacey, yea he might not want to have other girls, however he'll do it if its needed for their happiness, he resist, but his resistance has been crumbling over the Acts, while Lacey has been making him suffer he has grown to be more and more willing to participating in sexual activities with other women, that is literally corruption, Lacey has been eroding his resistance to her plan away corrupting him to be more willing to be involved with other girls, even if she isn't exactly happy about it she goes along with it, why? Because the notion people only do things they like is naive, compromise is most often than not the outcome.
As I said it before the notion that Lacey want to corrupt the MC isn't about anything bad for them, its about compromise, she want him to become more like her so that they'll better handle their shared mental issues and relationship issues, so is she happy about it? No, but she feel she has to do it if she want to keep her husband, whom she claim she couldn't live without and she couldn't leave even if she didn't love him. Its for the same reason the MC has over and over went for Lacey's plans, because he compromise its the only way to try to make their marriage keep working.
The story is about them overcoming their jealousy to retain their love, Lacey while being corrupted has still that much to deal with, and the MC has to become corrupted so at one point he can overcome the jealousy, corruption enable them to be able to achieve that goal, as it allow them to overlook the circumstances that would trigger their jealousy, because jealousy is entirely a normal feeling to have, the MC he is naturally completely entitled to his jealousy he has extremely good reasons for it, Lacey not at all considering what she has done, but in the name of love their relationship is problematic enough that the only way to make it work is to overcome their jealousy, if they can't they are doom to misery.
If anything this story story stand for isn't them breaking up at the end, their might be bad endings where it happen and they'll have to deal with the dire consequences of such outcome, but this story is very much meant to be one where they are meant to stay together at the end, and it can't happen so long as they suffer so much misery, so they need to eliminate the elements causing said misery, and in the short term jealousy is one thing they need to lose to be able to be at some degree of peace, is it ideal? No, is it necessary? Yes, would it be better for them to not be in a toxic relationship? Sure, but its not so simple when they are literally mentally intertwined together, they can't be separated right now, maybe one day in the distant post game future, but not right now.