I could ask you the same thing though Raithlin: Why is it that some people in every culture find others of the same sex attractive? The fact of the matter is that there will always be homosexual people, we may as well accept that and give them the rights that they deserve.
You could indeed, and I could give you my answer, but it would be based on my [biblical] values, and you may not accept it [I don't presume to know your value base, but am making an educated guess here].
As far as same sex relationships not lasting as long (assuming that's true): maybe when your lifestyle is shunned by society as has been for time immemorial, and you're forced to have secret relationships, it's hard to make long term commitments. Then when legislation suddenly changes and you CAN have an openly gay relationship, you're not accustomed to this way of life.
Being flogged/burned at the stake etc. tends to discourage people from being "open".
These numbers were taken from a study done in Sweden - easily the most homosexually open country on the planet. They (the laws) have been for quite a while too. You're also flying in the face of many long-term relationships (Take our old friend Elton, for instance) - despite lack of legislation.