Heterosexual marriages are at 50% divorce rates so how could you claim gay marriage is any more unstable than a "normal" hetero marriage? Your argument is predicated on stereotypes of gays not being in monogomous relationships when in fact historically they are as likely as heteros to be in monogomous relationships as they mature, just like heterosexuals.

I would say both are bad, but gay marriage would be less damaging (2 parents are better than one) if gay stayed married at a substantially higher rate. The problem is that there is no reason to think gay marriage will not have a divorce rate that is the same at regular marriage so any benefit would be a wash. Dealing with divorced gay parents seems to be a little harder for kids deal with. (Yea that's my dad with his new boyfriend???) In addition how do you work out custody when one parent is the biological one. I guess the non-bio parent would have to adopt- then you have to deal with all that. The first gay marriage in the US was followed by the first gay divorce. There is no reason to think gay marriage will do any better than reg marriage so there really is no benefit.

the right to marry isnt based on the ability of a marriage to contribute to anything other than your happiness or persuit of it.

Says who? Lets logically extend this out to incest and polygamy. If society has no say then why are those practices banned? If societal harm is removed from the equation then lets promote polygamy as well. Can you make a case for why incest marriage is wrong and gay marriage is acceptable? If you can bridge that gap then gay marriage will be on it's way to victory. Go!

there is nothing there to allow the law to keep anyone from marrying anyone else thus laws made to prevent gay marriage are by nature not constitutional and the first challenge such laws face in supreme court it will be shot down as unconstitutional.

The 10th amendment says otherwise. The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people. The only way it gets to the SC is if follows the tract of abortion "rights" which really do not exist. The right to privacy was a stretch and to get abortion from privacy is really a stretch and is generally seen as sloppy work by the SC. I doubt gay marriage will follow.

What rationale can you come up with to support keeping gays from marrying knowing whatever you say must apply equally to heterosexuals?

Gay marriage will hurt society by destroying the traditional family unit just as divorce did, just as polygamy would, just as incest marriage would.