Why are my friends leaving me out?

Ok well I'm gay (and male) and I sense that that has something to do with it. I'm seldom invited over to other peoples' houses, whether they are girls, boys, and I have never been invited over to a sleepover with other boys or girls, whereas everyone else does it at least twice a month very casually. I've never been invited to someones house before a party to hang out before the guests arrive. Even the people who are meant to be my friends never tell me when something important happens in their lives, like a breakup or something. I really tried to become friends with these guys who all hang out together and do all the above but I'm still just as isolated. It's terrible and I feel like I'm always out of EVERYTHING. Help me, please, why are my friends doing this? What can I do about it?