I've found myself going through the same sort of thought process sometimes, and I really don't know why. I've been bisexual as long as I can remember, and I don't think it's even a conscious thought process a lot of the time. But while I have no problem seeing two men in a happy, fulfilling relationship, my mind seems to have some kind of innate problem with two women having one like it. It might be the generalized way lesbians are usually portrayed in the media (unhappy endings, lots of angst, etc.) but for some reason, my mind balks at the idea of a happy, loving relationship between two well-adjusted women. (I refrain from using "normal" as it seems way too dependent on an individual's own definitions).
And this was no help at all, was it? Lol. You're not alone, though.