To back this up for a minute...
This is exactly the opposite of my point... it's actually the point you are arguing. You are making the case that someone's religious belief's and sensitivities should come into play when we decide what they are aloud to do. Murder results in real, tangible harm to the murder victim. That is why it is illegal. Adultery, using the lord's name in vain and illegitimate children are not illegal... even if they do offend the religious sensibilities of some.
THAT was the point... simply because something may offend someone does not mean it affects their life. If I punch someone or kill someone, that affects them. If two men decide to engage in a sexual relationship, name one tangible way that it affects anyone else's life, (other than making them uncomfortable). There aren't any because it doesn't.
Actually, adultery is illegal, though it is hardly the capital crime it once was. At one time in this nation’s history, a guy could be fined and/or jailed for committing adultery. Now, it’s more or less reduced to a “tack-on” foul. Basically, you won’t get arrested for it; but, if you do some other crime or have some legal dispute (particularly domestic ones like divorce), adultery can be added to the mix. And, in the military, committing adultery is a violation of the Uniform Code of Military Justice and can be grounds for separation.
As for illegitimate children, having kids out of wedlock doens't just affect those kids. The mothers of those kids who, more than likely, will raise them with little-to-no aid from the father, are far more likely to live in poverty. Having to work multiple jobs to support those kids means less supervision and more time for those kids to get into mischief, hang out in the streets, get involved in drugs, gangs, and crime.
With regards to the two men having sex, as I said to Straw Man, if those two men happened to be brothers, I seriously doubt people (even those don't think homosexuality is wrong) would be demanding that they be allowed to do that, much less engage or attempt to be involved in gay "marriage".
There are lots of things we could legalize, if the primary criteria is that it simply not directy affect anyone else's life: Incest, bestiality, pedophilia, just to name a few.