Why are my friend relationships limited, making me feel like im not a part of anything?

I'm 15 and possibly gay though i'm trying to act otherwise. I have many female friends and like 5 or 6 girls that are like my best friends if thats an appropriate term. I have no male friends, however. What troubles me is that these girl-friends of mine only limit our relationship to seeing each other at our houses once in a while and maybe going out for dinner or at a cafe or something. Never do they reach the sleepover phase or the all night dvd night kind of thing, which they do with eachother(only the girls). Same with the boys. Its like the boys are my friends but dont wish to take it any further like "come over to my house" when all the other boys meet up! Its just a "hi, whats up, bye". Ive never been to a guy sleepover and seldom have been invited over to their house. And I feel as if Im in the middle of everything and so many different meetings, sleepovers, outings are happening around me and im just not a part of anything. I mean, even the girl-friends I really love scarcely invite me out with them or ask me to sleep-over. It worries me that it would seem weird to sleepover at a girls house when the parents know about it(not knowing im gay). How can I become part of all this? How can I be close friends with guys and like really be involved in everything? It makes me rather sad, to think about it. Thanks everyone for your help!!!