Though same-sex activists would like the debate about the redefining of marriage to be about equal rights for adults, it is not. There are three different parties to the discussion, not just two.
The core of the discussion about redefining marriage is primarily about whether society needs a social space where the child’s natural right and desire to be raised by its two, biological parents is upheld.
Any sensible debate has to admit that children have rights too. They are naturally born wanting to be loved by the two opposite-sex people who gave them life. Also, children, on average, have the best start in life when raised by their two, biological parents. No amount of social engineering can change this.