Obviously there are countless denominations out there. Which is why discrimination ought to be analyzed on a case by case basis. If there's a church out there that says gay people are demons on earth, then marrying them clearly effects their expressive message.

However, many churches allow and permit sinning of many kinds. They do this because there is an admission that every human being is a sinner and that we must acknowledge that fact before we can become one with god.

Churches, as I stated in a different post below this one, often "sanction" sin, after a fashion or at the very least, permit it. This happens in case of divorce or premarital sex and a number of other issues I can't think off hand. I go into a lot more depth in the other post, so you can check there if you want, but you seem quick enough to get the gist of it.

So saying it's illegal to ban gay marriage is not altering the expressive message of a number of churches. If anything, it's upholding it.