If I'm understanding you, your argument is that the relationship could be abused by those of the same gender - they would marry and get the benefits.
But this can already be done with heterosexual couples. Yes, it would make abuse even easier, but if people intend to abuse that, they will think of a system under current marriage structure to do it.
Is it really worth depriving marriage rights to an entire group just to make marriage abuse (which, as far as I know, isn't all that common) a bit harder?
I believe it word become
increasingly common, if legal. As you've pointed out, it makes it easier. Personally, I'd love to have my father's pension for the remainder of my life. If there would be a legal way to collect it, I might consider it.
The basic argument against adult voluntary incest is the genetic implications; those simply do not exist with same sex couples. That's a matter of biology, not morality or legality. So, okay, we continue the ban on relative marriage, I still feel that this would be a problem. Are we suppose to get the government involved in determining that each relationship is a physical one.
What if I "marry" some old guy, perhaps pay off his current bills, so that I can get his pension for life? What if he's 80 and I'm 25? I can see a gigantic situation for abuse.
The concept is not uncommon; people do "sell" their life insurance rights in return for cash, especially if they have no heirs.