Corruption means they will go farther when they do engage; this does include other partners/multiple partners/kinkier acts. This is especially true for the school bathroom scenes.
Inhibition dictates how easily they are tempted into sexual activity and clothing choices, yes.
Obsession will not block acts with others. If you want to do that you have a couple of options: keep Corruption low or apply the mind control suppression named "Wild Oats". It prevents sexual interactions with anyone other than Billy. You can also choose to not activate the unisex bathroom -- most of the scenes with other participants happen there since Billy walks in and discovers the activity in progress.
There's not NTR in the game as Billy has no love interests. That said, should you wish to avoid seeing other males get some, limit Corruption to maybe 40 -- it varies by woman as their fetishes and interests (and thus rendered scenes) differ. The women most prone to other partners include Kelly, Samantha, Mary, Allie, and the principal.