One example of a stupid heterosexual couple does not mean that every heterosexual couple would treat marriage in the same manner. You don't like homosexuals being painted with a broad brush, so please don't do it to heterosexuals either.
I would like to think that most people getting married, whether they be homosexual or heterosexual, aren't standing there thinking "well, if this doesn't work out...
whatever..." I would like to think that most marrying couples (again, regardless of sexual orientation) go into it with the intention of it being a sanctified, forever kind of a thing. I know that when I married DH, I married him witht he intention of being married for life. We're going on 18 years, so I guess we're doing OK.
Pointing out the flaws in behavior some heterosexual marriages doesn't make any kind of argument for homosexual marriage because homeosexuals are human beings who might be capable of having the exact same flaws.
The argument for homosexual marriage is equality, not "the heterosexuals are screwing it up so bad that the homosexuals should have a chance to see if we can do it any better".