When a woman says "My girlfriend", it could mean just a lady friend or a lover. Why doesn't the same necessarily apply with guys saying "My boyfriend"?
It can apply in the proper context, but as a boy friend, because in these circumstances you use two words instead of one. It's just an unusual way of saying it... Generally speaking, most people would be aware it could cause confusion, so they'd phrase it differently.
Because society is more homophobic when it comes to male-male relationships than to female-female ones.
Because it's uncommon for men to be referred to as boys than it is for women to be referred to as girls. (not counting insult type stuff) So that transferred over to the other thing, boy friend / girl friend.
No it does the same thing as girl friend. It's just gross you out less when girl do that to another girl.
We're used to referring to any female as girl, regardless of her age. Even if a woman doesn't like it. We're used to respecting the fact that some guys/men view being labelled boy as insulting. Therefore we tend to avoid it. We've long grown accustomed to women being more open with their non-sexual romantic feelings towards family and friends. We stopped accepting this same behaviour between men a long time ago. Some people are homophobic and therefore anything potentially gay is wrong. For a man to call another man his boyfriend might be seen as homosexual so guys avoid it by saying guy friend instead. Some women will also avoid being mistaken as gay for the same reason by using other terms too.
Well, it is pretty hard to cheat when you as a guy put a gender on the friend. Or maybe it is because society paints women as whores and they have to report clarify to their male overlords and society at large that the friend is female to prevent punishment.
Double standards, just like how girls in a group of guys can be referred to as guys, but the opposite is not allowed.