Some people don't take Marriage seriously. To Celebrities it's just another scheme for publicity, but it's as legal for them as it is for any other straight couple. Yet they go in to a marriage knowing full well that they will be ending it soon at some crucial time: when the publicity is needed, when they are between movies, or between albums. Britney Spears' few-minute-long mariages are fully recognized and legal. And lots of heterosexual people who aren't celebrities have a similar lack of respect for the Sanctity of Marriage.
So why should those people get to shit all over Marriage and take it for granted when there are many other people who would respect it, but who can't qualify? There is no good argument for not giving Marriage rights to Gay people. In fact, with Britney as an example there's no good argument for not giving Marriage rights to kitty cats. There'd certainly be more dignity and respect.