NonprayingMantis: because gay marriage is so topical and controversial at the moment, the implication of the billboard is that the reason people wouldn't like their dads husband is because he is gay and there is therefore something intrinsically wrong with gay people, rather than (if it were just 'wife') because they are annoying.
consider how acceptable it would be to put up a billboard that said something like
"I really like my dads new Maori wife... Yeah right"
wow thats not what I got out of it at all, interesting how some peoples minds perceive things.