Loki:
Loki doesn't have much chance to get jealous, since he's not allowed out and about. However, sometimes another SHIELD agent will hit on you, or you'll be allowed some time out with supervision, and he will use his magic to suddenly make the guy realise he's surrounded by a supervillain. Or rather, several of him. All glaring at him. The guy will usually run.

Tony:
Tony will generally walk up to you and kiss you passionately, before pulling away and slipping an arm around your waist. Then he'll ask casually who "this guy" is, giving him the full time to realise that this is the one and only Tony Stark.

Steve:
Steve will get really mad but try to hide it, and as soon as he can think of an excuse to end the conversation, he'll use it. If the other guy's being a jerk, he'll stand up for you.

Clint:
Jealousy has to be Clint's biggest flaw when it comes to you. In his eyes, anything from threatening his life to punching him in the face is acceptable. You're his and the male population better learn quick.

Bruce:
Obviously, he tries to stay away so he doesn't Hulk out. But if the guy gets too annoying, you point out that you're the Hulk's girlfriend. When he doesn't believe you, you point and Bruce waves. The guy leaves fast.

Bucky:
They see you; they think This is a good idea. They see the super soldier with the metal arm at your side? They think Maybe not.

Pietro:
Pietro's not afraid to do as he likes. He'll appear at your side, grab you, and drop you off somewhere else. Then he'll come back and punch the guy for good measure. Well, he didn't see that coming.