The people have a right to decide that a contract to violate the rights of a third party (e.g. a "hit" contract) is unenforceable.
The people do not have a right to decide that certain individuals have a "special right" to enter into contracts that are unavailable to other people (e.g. a zoning prohibition against home purchases by a disfavored ethnic group).
Simple, really.
This does not apply in this case as any homosexual has the same right to marry a member of the opposite sex that any other citizen has. Nice try and thanks for playing.