Any Christians on here that belong to churches that advocate for same-sex marriage?

I'm a Jew but we have similar issues with us. Just because a certain synagogue or church chooses to ordain a gay wedding does NOT mean that it's what God wants. An individual Rabbi or Pastor that chooses to marry gay people are going against what the Bible says and thus they are to be avoided.

However the message is to love people and not to judge them. This goes for gay people as well, as believers in God no synagogue or church should ever harass or discriminate against you. We are ALL God's children.

I recommend you find a church that allows and accepts gay people, but at the same time does NOT go against what the Bible says. God only wants a man and a woman to be married, that's it.

If they are they are liberal and worldly churches that classify as the Laodicean type church: “To the angel of the church in Laodicea write:

These are the words of the Amen, the faithful and true witness, the ruler of God’s creation. 15 I know your deeds, that you are neither cold nor hot. I wish you were either one or the other! 16 So, because you are lukewarm—neither hot nor cold—I am about to spit you out of my mouth. 17 You say, ‘I am rich; I have acquired wealth and do not need a thing.’ But you do not realize that you are wretched, pitiful, poor, blind and naked. 18 I counsel you to buy from me gold refined in the fire, so you can become rich; and white clothes to wear, so you can cover your shameful nakedness; and salve to put on your eyes, so you can see.

How could a person/church that knowingly disobeys God call itself "Christian"?

As you say, it would be quite difficult to pin point where support comes from. Certainly not from the Bible!

Quakers perform gay marriages.

At least in Britain.

anyone who rejects the bible is not Christian

they may be churches, but they are not Christian

the bible is very clear in its prohibition of sexual sin of all kinds...New and Old testament