I agree with you, and while I can't speak for others, I can tell you what I observe.
It seems that people who dislike same-sex relationships are stuck on the notion that a male/female union is the only "proper" one to have, with the male being the dominant (not just sexually!) partner; the female being the "helpmate."
Any relationship that challenges that model, even a hetero one in which the female is dominant, causes an outcry. All of a sudden, you have to rethink all your neat little categories, and let's face it, that's not fun to do.
Furthermore, in a same-sex relationship, who's the dominant partner if they're both male? Who's the helpmate if they're both female? Oh, the quandary.
I believe that way of thinking relies on very unrealistic stereotypes of how men and women are. It's more challenging to accept that we are flexible, difficult, sometimes maddening, but always entertaining beings.
If you find someone you love, and they love you back, you're lucky. Simple as that.