This is exactly what has happened with my friend, whose sister decided she was a lesbian and ended up "marrying" another woman. The sister holds the entire family hostage, threatening to deprive them of her presence unless they constantly offer praise for her lifestyle. Disagreement results in tantrums, and more than one holiday gathering has resulted in the lesbian couple storming out.
When they’re constantly sticking it in your face and demanding approval, I believe it’s because they know it’s wrong, and want you to validate their incorrect choice.
I have gay neighbors. They mind their own business, they’re not “in your face”, they’re friendly, and they don’t demand approval. Which is why we get along so well.