For now, I will just say this: There are two issues here: (1) freedom/rights, and (2) state action. Different rules apply. But either way, religion need not enter into it.
Hetero-relationships (individual freedom) have always been supportive of a better future society by producing contributing members of society, so states have acted to encourage stable, creative families by establishing secular marriage (state action).
The Left sees no need for a stable family, since the state should provide for everyone in need. Given that approach, I'm not surprised things have slid to you supporting giving marital endorsement (state action) to homo-relationships (individual freedom)...all while the Left argues there is no slippery slope.
There is no reason for the state to endorse any relationship. Marriage has always been about the children, not about the couple. The Left's fundamental misunderstanding of the purpose of secular marriage has led to them trying to redefine marriage.