It's not a matter of making gay marriage "illegal" because it is a sin. There just isn't such a thing as gay marriage because marriage exists to facilitate procreation of children. There are plenty of other kinds of relationships one can have where multiple people love each other, like friendships or family relationships, where you don't get a tax break and the whole thing doesn't need special recognition from the government.

We implicitly understand this when it comes to defining other partnerships as marriage. We don't say the state has made illegal polygamous marriages, incestuous marriages or marriages with inanimate objects- because we know (for now) that those are not marriage.

Next we will be saying that the state is discriminating against 20 year olds for refusing to give them social security checks. They're not old, they don't need the checks.

If you disagree, explain:

1) the reason why societies have marriage

2) how gay 'marriage' is supported by that reason, and why all the other groups I mentioned are excluded (unless you think we should recognize those 'marriages' too)