how do you respond to someone like that?

And my old friend is dating a girl who also happens to be my SOs best friend since highschool. She's open about it on Facebook and this lady goes off about how she's doing wrong being with a woman and she knows it's wrong and that she needs to go to god and repent so she doesn't go to hell. 

I adore this old lady okay, even though she has completely opposite views from me. And she's just going on and on and on about how being gay is wrong and it took her like 5 minutes to realize I wasn't responding. She looked at me and I just said "I'm just happy that she's happy" like kinda rude. I didn't mean to, but I snubbed her. There is no point in trying to explain because she's old and set in her ways and just ugh. That's all she knows. She's from a different time. 

How would you respond to that respectfully?? Let's say you're an LGBT ally and someone you respect is trashing that and you don't want to make them angry at you?? It would have been ENTIRELY different if my friend was there being told all that bs. I try to be tolerant unless it's directly affecting someone.