I have a girlfriend and no friends. I seem to be far on the edge of the friends spectrum. The general idea I feel society has about friends and what a friend should be is only something I feel comfortable with in a significant other. If a person isn't my significant other, well all I really want is for them to be an acquaintance. I don't really want to talk to them, I don't want to hear what they have to say, I don't want to hang out, or go to a movie with them. All my "friends" prior to my girlfriends were just people I lived with, shared a room with, spent time around because of work, or school. A movie is often an excuse to hang out with friends, for me, a friend was an excuse to get out and see a movie.
The parallel play thing someone else mentioned really speaks to me. As a child I did have friends, everyday after school we would go play in the woods. Friends would come over and we'd play with toys. The thing is, I was never connected to what they were doing, I was always playing by myself, they just happened to be near by. This is still the kind of friendships I would like as an adult, but it just seems like this sort of thing doesn't qualify as a friendship anymore. It only really worked for me when I had house mates. I enjoy their company, I don't get lonely having them around. But I don't want to actually talk or engage with them. I want to ignore them and I want them to ignore me, and I'm a happy clam.
I enjoy the company of others, I just need the freedom to be in my own world, completely disconnected to whats going on with theirs. Being connected in that way is very very intimate to me, something I can only enjoy doing with my girlfriend, but everyone who isn't her, I don't want to be in their head and I don't want them in mine. So yeah, I haven't actually done something recreational with another adult other than my girlfriend for 5+ years now. And I love it.