I had an interesting experience today.
My roommate has been somewhat harassed by a guy in our dorm building and she's been trying to avoid him, but he's on our hall and we had our first hall meeting tonight. I had a super busy day, didn't get dinner before class, then got on the wrong bus on the way to the meeting, so I was starving and running late. When I got there, I came up behind her and somewhat startled her by laying my head on her shoulder and giving her a side hug then explaining my situation and asking what food we had in our room (because the meeting ran after the cafeteria closed.) When I walked over this guy came over and hovered.
Anyways, as we were leaving this guy came up to her and started talking to her, she was looking super awkward, and I hung around right there because she told me that he was harassing her before school started. He insisted on hugging her to part ways. The thing is they've never met in person, the harassing was on Facebook, so this was super weird. Once he was out of earshot I asked her what that was about. She said apparently he'd been staring at her the whole meeting, and as soon as I had approached her so affectionately/comfortably he walked right over and hovered really close. He wasn't doing anything until I got there. That's why she was so startled when I came, she thought I was him at first since I came from behind her.

So what's up wit that? Was I threatening to him? Was I making him feel less manly? It was weird. Either way I think he was jealous of me and that made me pretty happy (as in, made me feel like a guy), I just hope he stays the hell away because he just wants to get in her pants along with every other girl on campus from what I've heard. Maybe if he knows I'm with her he won't creep so much.

Could be. Honestly, if there was a girl I had my eye on and I saw some dude getting that close to her I'd get pretty jealous and, if I felt confident enough on that day, instinctively done something like he had done.

It seems like he views you as competition. He probably saw that she was comfortable with how affectionate you were so he tried to be affectionate too since all his advances on Facebook (if that's what the harassment was) didn't work.

I wouldn't say you threatened him or made him feel less manly but he probably did see you as competition. A guy I work with sees me the same way and we've never gotten along because of it, he's jealous of how the girls are around me (I'm completely stealth although idk if that makes a difference here).

Wow. The guy sounds like a creeper. It does sound like he was jealous! Congrats!