There should be secular reasons to support laws. There is no secular reason for this.
First, it is not the proper topic for the Federal government. This is States issue.
Second, the only reason for government to endorse marriage in any way is because marriage encourages (i) procreation and (ii) stable environment for raising productive humans, both of which benefit the economy of the State.
The State should not care whether, and should not endorse me, just because I found a nice girl to spend my life with. There is no point in that. Same for other long term personal friendships. There is no point. But, if those long term relationships help society (e.g., by helping the economy), then the State should care.
Endorsing behavior that is counterproductive to procreation, at a time when we are experiencing a population crash, is counterproductive, and will punish future generations of America, and the world, since we are the last best hope for global freedom. Why not remain silent on such behaviors?
Do homosexuals have a right to engage in homosexual acts? Of course, because there is no victim in those acts. My religion can condemn those acts as being counter to an ordered life in relation to God. But, Congress/legislatures shouldn't care about that. And, I don't think a legislature should outright condemn a victimless act. However, why endorse a victimless act when that act is counterproductive to a long-term healthy society (e.g., by being counterproductive to the above two points, and therefore to economic growth)? Why not just remain silent on that act? Why waste government resources to try to encourage it? Seems weird.
It is just another example of the Left not caring about future generations. Want to do something now?...borrow from future generations, without any regard to the damage it does to those future generations. From this issue, to abortion, to deficit spending...that is the general policy of the Left to disregard the future generations in favor of the now.