Fight him? Alpha up. If you have the height and the muscle mass all you're missing is the balls. Stand up to him and tell him to go f*ck himself. His girls thong was showing and you let her know. No infraction on your part.
This... Seriously. And then you had a story about how you tried to protect a female from creepy guys, then her boyfriend tried to fight you because he felt inferior and you bitched him out by calling his bluff. Thanks for the story.
Next time you can be like: Hey this is going to be really awkward for you but your thong is sticking out. I figured I'd let you know because I'm brutally honest. Do you have a boyfriend? No, ok good because you females have a bad habit of not covering your backside and the last time... And I really didn't want to be in that situation again. I don't like hurting people.