That is exactly the issue, but here is the catch to the whole thing.
Marriage (the religious institution)-
if I as a member of any/or all churches believe it is permissible (namely that my personal god permits it) then it is allowable under god. I.E a religious marriage is ok. Most people other than fundies would have no real issue seeing it this way. Because if I can't follow my own beliefs then why should you be able to follow yours, or vice versa (no religion gets to tell another religion what they can/can't do)? Basically I get the spiritual benefit of the marriage, but no legal parts.
So guess what? You can not deny anyone the right to a religious marriage if they can find a priest, monk, or other "holy" figure that will marry them. - I am ordained and I will marry whoever I want.
Marriage (the legal institution)-
if I as a member of society marry a person we share all things (money, property, etc) unless things are spelled out in a prenuptual agreement we have to determine who gets what when we split. This is why divorce lawyers are usually involved, and why prenups even exist. Legally anyone over 18 (an adult) or who has the consent of their parents can enter a legally binding agreement. Look at any other legally bindig contract and it will have the same kind of things as a marriage- we agree to the following terms....blah, blah, blah. This is a contract- the government itself issues the contract (you have to get it from a government employee) and a ship captain (?seriously, why?), or a justice of the peace can perform this. Basically I get the legal benefits of a marriage, divorced (yeah I meant to use that) from the holy/religious aspects of it.
We have this thing called seperation of church and state...no state sponsored religion...so see point above- if your religion does not deny it, laws don't deny it, and it doesn't say kill puppies or children, then it is permissible. The only beef anyone can throw at this is that sodomy laws exist on most state law books, but how many people get arrested for that if it is consenting?
Problem solved, there is no legal, ethical way to deny people the right to marry another human...if you want to talk animals, that is another story as most cultures (most) look down on that.
Polygamy would fall under this same thing by the way...multi-party contracts do exist. If everyone involved is cool with it and knows what happens when the divorce comes then why not?
On the only flip side- degredation of family values...I'm not legally married...is my son really missing out on anything? There are stats that gay couple stay together more frequently (stats have been run on this)- think about it...if you can be killed (yes, it's happened) just for asking someone out, wouldn't you stay with someone rather than be trying to find another relationship. And ultimately, the only difference in upbringing should be that they are probably more open to other peoples. Lets face it evil people raise usually raise evil children, it doesn't matter who they hump.
