No, if heterosexual marriage is threatened by anything, it's by heterosexuals. Famous heterosexuals in particular. We see them grinning from the covers of gossip mags, celebrating wedding No. 9 or dissolving marriage No. 14, or just having a hot fling with whatever good-gened, white-toothed cretin is the flavor of the season.
People don't get divorced because Demi did. That's not the point. But because the culture attaches no particular stigma to divorce or catting around, our pop-culture heroes don't even have to pretend anymore. Say what you will about gay marriage, it's nice to see someone taking the institution seriously.