Here's some food for thought: Before the rise of civilization (~10,000 years ago), humanity operated in small tribes. The children were all raised predominantly by the women while the men secured food sources. There were no neat, pre-packaged nuclear "families," yet humanity seemed to end up okay. In fact, it most resembled being raised by a bunch of lesbians. This is in fact how it's done in many primitive tribes in africa. Last I checked they seem to have a lot fewer issues than we do.
I think people are either religious or insecure, or both. They've hinged their identity on "husband," or "wife," and they don't want to share that with gays because it damages the bullshit identity they've formed for themselves. So basically just selfishness.