As far as I've seen he hasn't done that again there after. He's been a scumbag plenty of times though, but not like that at the very least.
There's a scene with Miku that's arguably worse.
And that scene is never stated to be under an outside influence, so as far as we know, he is fully in control and is just taking his scumbag level to new heights.
I think that is precisely the problem many people have issue with. It's not that Sensei's a scumbag doing shady stuff, it's that he and every other character verbally recognize and acknowledge it and this apparently makes some people uncomfortable. For me it's better this way, as I find it refreshing, but to each its own.
Also, games that cater to that kind of thing are usually honest about it in the description page, where here you get blindsided by it, and also are not given a choice to do it or not. This creates a loss of player agency that can be unsettling.
I think it boils down to an expectation that the game won't acknowledge what is being done as bad because it is a game, so for a game to fully recognize a bad thing for what it is in real life, bad, is not expected. The unexpected can make people uncomfortable, especially when the unexpected thing in question is seen as bad in the real world. For me, though, it just gets me angry at a certain someone in the game.
Most games that delve into the inherently morally negative fetishes, negative in that they are illegal to be done in the same context for real, ONLY delve into them. They may acknowledge it in the threads, but they don't acknowledge it in the game except maybe a warning in the startup sequence. This is because they are focusing entirely on the sexual side, not the moral side.
Lessons in Love doesn't focus entirely on the sexual side of these things and, in fact, barely focuses on the sexual side at all. It also acknowledges every act, whether good or bad, as what they are. This is one thing, as a writer, that I do enjoy about the game, it doesn't sugar coat, it doesn't ignore, it tells things like they are.