>Why do I care if my position on marriage promotes or discourages the creation of new humans? I guess you haven't noticed, but we have enough.
I disagree that we have enough, and some first-world nations are now starting to become aware that they've decimated themselves and that their national identity is now in real danger of extinction, not because of massive war or famine, but because they simply chose not to have children anymore. In my mind, things must be severely broken to make this condition possible.
>What is the ideal balance of masculine and feminine traits in a person's upbringing
The masculinity and femininity contributed by one full-time parent from each sex.
>how did you deduce this ideal balance
Nature has deduced it for you, and every respectable shred of data in existence supports and admits that children fare better in loving homes with both biological parents. This is instinctively understood by persons not fully brainwashed.
>how does a heterosexual union achieve this balance
The sexuality of the union is heterogeneous, which is to say, there is a male and a female. This provides 1 masculine figure and 1 feminine figure as the major lodestars in a child's life, and their interplay teaches the child how to interact with his peers of both sexes.
>Can you point out why a homosexual union does not achieve this balance?
The sexuality of the union is homogeneous, which is to say, there are two parents of the same sex. This provides lodestars of only one sex, and their interplay cannot as effectively teach children to interact with peers of both sexes because there is no sexual dimorphism among the parents.
>Shall we also ban divorce?
We should not outright ban divorce, but I believe we should make it much stricter, and I believe the proliferation of no-fault divorce and the devaluation of family court is another milestone on the now nearly-complete road to social collapse. Divorces should be much, much more difficult to get, and much rarer. The current rate of divorce devalues the marital institution as a whole, which is not to say that the acceptance of homosexual marriage doesn't devalue it further.
>Since when is sexual partnership the fundamental unit of social cohesion
Since always. Hunter-gatherers were not civilized or socialized at the level which is meant when people talk about human civilization.
>Furthermore, I suggest that intolerance of different sexual preferences threatens social cohesion a lot more than the preferences themselves.
I disagree. Acceptance of sexual deviance is a threat to the cohesion of a group on many levels. I do not accept the narrative that those impulses are uncontrollable and that it's unkind to suggest discipline, just as I don't accept that narrative as applied to other anti-social behaviors, like theft, that are considered negative. Society must invoke the discipline necessary for its survival where the individual fails to do so.