Tony
Gets insanely jealous and even a little angry about it. He'll give the guy a death stare and make out with you right there to prove a point, but also he enjoys it. He walks away holding your hand and personally threatens him to back off.

Steve
Steve will get annoyed. He'll pick you up in a hug and spin you around, placing a kiss on your forehead after to make his point clear. He walks out with his arm around your shoulder and talks about what they said after it happened. "Well, he tried, but you're all mine!"

Peter P.
Peter backs off a bit, not wanting to get into any trouble. At first he thinks the guy is just being harmless and just complimenting you until he realizes that you're getting a little uncomfortable. He kind of snaps a bit and tells the guy to screw off, you're his. He kinda regrets the passive aggressive reaction when he receives a dirty look but sighs with relief when he finally goes away. He hugs you and kisses you, looking at the guy and hoping he sees this.

Thor
Thor will directly confront the guy and ask him what he thinks he's doing. If you weren't there he probably would've broken out into a fight already with him, but you broke it up before things got too out of hand. The guy retreated and left you two alone. Thor walked out with you holding your hand and kissing your forehead.

Clint
Clint won't say anything at the moment, but just before you two walk out he says he wants to stay behind for a sec. He just had a nice, friendly talk with the guy telling him that if he ever tries to flirt with you again he'll have his head blown off his body.