The key to understanding this debate is to understand that it has little to do with homosexuality, marriage or equality.
Marriage is a normative statement about human reality, the human person and society. Rooted in our creation as male and female, it asserts the reality of the sexes, the difference between them, their complementarity and the need for their mutual co-operation. It says that as humanity comprises the sexes, we need mothers and fathers, and from that, there is a natural family with natural rights.
"Genderless marriage" (for that is what we are actually talking about), is a statement that contradicts that. It's implicit assertion is that gender is a construct, moreover an undesirable one that needs to be abolished.