Marriage is an institution.
An institution is a way of life (which can be socially represented in many ways) that society concedes to itself in order to warrant its continuity.
Every institution discriminates, because there will always be some people which have the right conditions to be accepted by a whatever institution, and other people do not. For example, a gentlemen’s exclusive club (i.e., an institution) is supposed not accepting female members; this means that some people fulfill the right conditions to be members of that institution in particular.
If an institution does not discriminate, in some way, it follows then that is not an institution at all.
If subjective “love” is a “right to marriage”, then, in theory, every form of “love” has the right to be part of the marriage institution. And if marriage institution does not discriminate any subjective “love” (does not discriminate some people), then it follows that marriage institution ceases to exist.