For starters, the spiritual lives of everyone connected with that church is in jeopardy...
And I heard another voice from heaven, saying, Come out of her, my people, that ye be not partakers of her sins, and that ye receive not of her plagues.
For her sins have reached unto heaven, and God hath remembered her iniquities
Revelation 18:4-5
The point there is that there are two distinct people : the people of God and those identified with Babylon. Even though distinct, the people of God in Babylon are to separate from the influence lest they approve what God calls wicked and get caught up in the judgment due the unrepentant Babylonian.
Maybe a little more direct :
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?
And what concord hath Christ with Belial? or what part hath he that believeth with an infidel?
And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people.
Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you.
And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty.
2 Corinthians 6:14-18
The one who approves what God calls sin is not a believer. To take that into a church and foist that on others is to raise an idol in the church. It's bad enough when it happens at all, but when it is going on in a church in direct violation of the teaching of the scriptures (which are supposed to be the foundation of the teaching of the church), it's confusion (which is what "Babylon" means). The point being that anyone who identifies with someone who is a rebellious, unrepentant sinner (like someone wanting to justify homosexuality and have others approve of it - much less under the banner of Christianity!) is not someone God approves of. It is clear that they put themselves in great spiritual danger. Even more, the one who pastors the church in that direction :
My brethren, be not many masters, knowing that we shall receive the greater condemnation.
James 3:1
The bottom line is if one is serving God, one will not approve of what He disapproves. Just that basic point of worship alone (regardless of the religion) should be clear. If one violates the principles upon which the faith is based, one violates the faith and at leasts mocks it. And if one believes God is the author of that faith, one has God to answer to.
