I agree with you, they are two different tunes, but never sung simultaneously in practice.
This was never about "treating people as lesser humans", despite the cawing hyperbole from the pro-homosexual-marriage faction...
This was basically a semantic debate.
Marriage, for most of recorded history, was defined as one thing.
There's a new style of relationship that's arisen in modern times, and that group wants to extend the definition of marriage to cover that as well.
Heck, in many countries, you don't even need a marriage, leg alone a civil union - simply being in a de-facto relationship (i.e. living together) will give you the same privileges (tax, medical etc.)
It was never about privileges (government's can't grant rights), but just about ideologies.
How can you have a rational reason with an irrational premise? If the premise is irrational, so is anything based on it.
So, no, there are zero rational reasons to oppose gay marriage, only irrational, bigoted reasons.