They do have a point - it does seem a little off colour - but part of my argument for gay marriage was that if it's no skin off your nose and none of your business, why shouldn't two people be allowed to wed?
Surely the same logic applies with two mates - though I'd argue they should sign a well-thought-out pre-nup.
As the Edge has said - it's the first wedding they've done they know for sure will end in divorce.
Maybe those who can't enter because a) they're female, b) they're gay or c) they're married, have greater cause to complain.
It would be interesting to see if the same people so offended on behalf of the institution of marriage when gay people wanted the right to marry are similarly offended now that two mates are set to tie the knot. Somehow I suspect that for many their true opposition was not to a threat to the institution of marriage, but to homosexuality itself.