I have this theory about sexuality being linked to evolution. The short version is -
The majority homosexuals/non-sexuals will not reproduce because they are unable to do so with a partner of the same sex/they have no desire to (of course there are exceptions to both of these). With a portion of the population not reproducing, population levels are maintained without the need to cull the existing population.
Makes sense IMO, and apparently there are boffins out there who agree with me. One reader sent in this great paper to Savage Love about the topic which I found to be in interesting read.
And if you are interested in hearing about the reader lose his anal virginity to his girlfriend and seeing a picture of his arse , the original letter is here.