I can see how - on a continuum - a species with a tendency for bisexual behaviour could be naturally selected for. This would result in stronger bonds, better co-operation etc. amongst members of the species. Rampant heterosexual behaviour and purely homosexual behaviour would be both be extremes of 'the norm'. This would, perhaps, go some way to explaining why we are all slightly bisexual and exist on the continuum.

I rather like irate_atheist's thesis, but then I am gay myself and prefer to think of my orientation as being part of the whole evolutionary process and not an aberration like, say, mental illness.
I CAN attest to the fact that I have experienced romantic love of Wagnerian intensity and feel every bit as fulfilled as someone with a hubby and gaggle of babies. As Steve Zara pointed out, homosexuality is too widespread throughout the animal kingdom to be aberrant. I think sexual orientation (and love) are very complex, multi-symptom (not sure that's the right word here) events that probably involves a score of genes and another score of environmental factors. I can "imagine" myself as a hetero mommy, but being a lesbian is just more (ahem) 'natural'. While I am sure there are lots of people on this list who would deny having any trace of same-sex urges deep down inside (and who I am to judge?) I still think it is accurate to suggest that the human race has an unmistakable bisexuality to it (I mean of interest focus, not only of reproductive mode).