You are conflating two very different things. About half of the gay men I've known were fairly or very masculine, they just were attracted to men. And I've known plenty of straight guys that ranged from feminine to downright flamboyant. The way you tell if someone likes men or not is either if they tell you or flirt with you, and if they're a man, then they are gay (or bi or whatever non-strictly-straight flavor).

Incidentally, most of the (non-trans) men I've known that were feminine or flamboyant were artists of some flavor, which tend to be a group with less inhibitions, so I would guess there are plenty of non-artist men that would show more femininity if they felt more comfortable with doing so.