The familiy institution, as we know it - heterosexaul, monogamous - is a 'de facto' state of affairs in our society. I agree that monogamy is not the only possible heterosexual family arrangement but 'our family' is monogamous. Of course, monogamy is in peril if we agree to see ourselves as a 'multiculturally diverse' type of human amalgamation instead of a 'Western', largely Christian nation.
On the other hand, there is no example anywhere in the world and at any time in the past of 'homosexual' families. We know that people used to seek pleasure in fornicating with children or humans of the same sex, or with animals, or corpses, or with themselves but such practices never turned into 'families'.
The 'homosexual family' would be something completely knew and probably impossible to sustain. Its only drivers - tax and other 'welfare society'-type 'benefits'.
I agree with what you said. I also think it is imperative a SECULAR basis for marriage should be the scientific realities of human reproductive biology (male + female) that renders the whole homosexual issue moot.