I’ve said this before.
Once you allow the one to be legal, you cannot win a legal argument against the other.
If two men can get married, how would you ever argue against 3 men getting married?
But also consider how an equal-protection based assault on heterosexual marriage at least partially takes out the basis for prohibitions on incest. Take the example of an 87 yo great grandfather who is a widower and elects to marry his 19 yo grandson, on his deathbed. Two males, even if within typically proscribed consanguinity cannot procreate. So the public policy rationale for avoiding incest is not applicable here.Normally when a grandfather devises to grandchildren or great-grandchildren a generation skipping tax may come into play. So what if grandfather or great grandfather brings the beneficiary into, in effect, his generation by marrying him? I hope, but I doubt that the IRS has seen this coming.