“As far as cousins marrying, so long as you don’t have it as often as the European Royalty did, it isn’t as risky.
Why should there be any consanguinity limitations for a homosexual “marriage”?”
For that matter why should you even bother calling their union a Marriage? There are no natural children(family) to be bound around nor is their any possibility that there ever will be.
As individuals they may adopt or have children with member of the other sex but in such cases it is rather clear which adult has parental rights & responsibility and there need be no union to bind that which was never naturally formed.
Frankly if they want to dance around and pretend their husband and wife in a sodomite relationship let them do it with their own resources and not involve the rest of us in their sinful affair.