In a symbol-manipulating and culture-producing species, homosexuals fulfill the function of outliers not bound by responsibilities of family and to a certain degree free from regular societal norms and expectations. As such they are in a position to dedicate themselves to niche endeavors which may have no immediate useful function for society, but contribute to the experimentation and innovation which keep human culture evolving and adapting. They are often able to take bigger risks and pursue less profitable career-paths compared to heterosexual people with a family. Having children/spouses be dependent on your income makes you more conservative in taking risks and experimenting with your perceived role in society. Thus you can find many homosexuals in fields which demand extreme immersion and individuality of thinking in order to produce results: art and science.
Anyway it's probably best to ask some gay people what they think about all this.
Sexual promiscuity and relative freedom, communal raising of the young, polygamy etc., are believed to be frequent behaviors (common in >50% of communities) in pre-agricultural human societies. The few remaining human communities that are in a pre-agricultural stage (hunters-gatherers) do follow this pattern, as do other social non-human primates (with exceptions). The percentage of communities with such communal sexual behaviour is species-specific. For example, bonobos and common chimpanzees are like this, but other large ape species have different behavior distribution (I'm thinking of mountain gorillas, baboons etc).
The theory which is more popular among ethologists and anthropologists is that when agriculture sets in, there is literal and metaphorical fence-building: My land, my harvest, my woman. Don't touch or I'll kill you. Besides, not only the "landlord"'s role (male head of family), but also the female's, becomes harder to replace if it's just the basic couple, rather than the whole village, who must work the crops and the house.
So the poetic image that goes along with this theory is that agriculture is the "loss of innocence" landmark for mankind. The pre-agricultural state can then be mythically transfigured into a lost paradise.
My personal experience, while not significant statistically, reinforces this myth. I used to have a business in a distant part of the world. There, I befriended many people that have a nomadic (pre-agricultural) history and were forced into "civilization" only a few generations ago. They are different, indeed. The women react very differently. The men react quite differently. The sense of possession is much weaker indeed. The sense of impermanence is palpable. I like that general feeling very much. Of course, with the Internet and globalization reaching every "civilized" human, the dominant mentality quickly takes over. Things are changing fast and even trace amounts of that innocent paradise are being lost and will vanish soon.
Wait,wait, i can´t learn so many english words in one day !