Marriage has never been just about the happiness ... of the individuals involved. ...Human societies need stable family groupings...

First, happiness and fulfilment of the individuals involved is helpful, and almost necessary, for successful and stable marriage.

Second, there is no proof that heterosexual couples tend to make more stable family groupings, or provide better care to the children. Though, I expect that time will provide the data needed on this matter.

Third, even though having their own child is almost infeasible for homosexual couple, there are surely a lot of orphans to be adopted. And a homosexual couple is less likely to adopt, since they have an option of nurturing their own flesh and blood. Add to this problem the possibility of a couple adopting a recently orphaned child related to one or both them by blood, and your will see that gender of the partners doesn't matter much when taking care of the future generation.

Fourth, gender identity can be ambiguous, psychologically, socially, biologically, in any and all senses of the word, with intersexuality as an example of this.