You are never involved in any intimate relationships with any of the long line of boys you talk about. You seem to have sex with all of them, though, and then you wonder if they are jealous, if they care, if they like you or someone else you think they might like. Now you're noticing some guy's erection while he's standing behind a printer?
Who cares? It isn't your business. He isn't your fiancé. You are not in a monogamous relationship with him, or anyone else. All you ever do is ask these "why would he do that" questions that are completely none of anyone's business.
If you want to know what these boys think, ASK them. Stop annoying others with questions that only these boys can answer for you. Or is it just attention that you want here?