To add to your points, in my own family my great aunt lived with another woman in what could only be called a gay marriage for DECADES. There was no infidelity that I was aware of, and they were two of the most loving and caring people around.
While I understand society's desire to keep the word "marriage" exclusive to one man and one woman, the idea of civil unions does not bother me at all.
And in a basic sense, it is irrelevant to me whether someone is born a certain way or not.
Lesbians tend to be more monogamus but they also represent (if I remember correctly) only about 9% of all homosexuals. Homosexuality interestingly enough has the same distribution from male to female as most sexual disorders where as men represent the largest portion of such populations.