Marriage is the oldest social institution we have that promotes gender inclusion. The fact is that SSM promotes an obsolete notion, that of gender exclusion on no other basis than personal preference. For example gay men exclude women as a class from their marriages simply because they prefer to. The classic definition of discrimination.
Good social policy is based on "best practices". The best practice in regards to parenting is for a child to be raised in a loving home by her mother & father who are happily married to each other. If there is a better practice supporters of SSM simply can't name it all they can do is hope others don't ask, "what is best for a child?"