Very clearly thus far they get aroused enough to have an erection. Momo does, mushroom girl does and now the gnu does. I didn't mention the fox girl because I've noticed she's different in that aspect and just fights normally for a reason, you know.
Let me fire back at you then, why have the characters have an erection when they're supposedly trying to kill Nene after getting beat up enough then? The gnu ends up sucking off Nene after getting beaten up enough, and Nene ends up fucking the characters after beating them up after they supposedly tried to kill her, that makes as much sense as you're trying to push it to have a logical explanation on a eroge. I'm just asking for a mechanic to be considered since I happen to like it, nothing else.
No need to raise pitchforks and torches.