Forget the unsustantied allegation in the last sentence, look at the middle bit, the bit about heterosexual unions being devauled if any other lifestyle is seen as equal, because that will come up later. How does it follow?

The same way that membership in a social organization is devalued when blacks, or Jews, or women are let in. It's a zero-sum game, as you said. If someone different from me gets what has been exclusive to my kind, of course it lessens the value for me. Perfectly obvious.

There you go. (Or, in Minnesotan, thuryago.)