I don't believe Mark is gay, not because of his flirty content. That would mean that all men who like to show it off are gay, or that all male strippers or models are gay, which isn't necessarily true.
That said, I don't see the problem with people shipping or joking about him being with other men. Fantasizing is pretty harmless. Straight men joke about being gay and hooking up with other men all the time even if they don't actually do those things. (Or at least, not ALL of them do these things. Some do on DL.)
Now, COULD Mark be somewhat attracted to men on the Kinsey scale or something? Who knows? Possibly. But there's no proof of that beyond people's wishful thinking.
I think he's just someone who likes to show what he's got and knows how to work his own fanbase for his own popularity and benefit, which is something most people with his kind of career would do.