It's either the one where they find an angel or the one where homer starts officiating marriages for gay couples. Unless there is another episode I am forgetting where Homer turns his garage into a church and wears a priest outfit.
I was leaning toward the latter because I only remember Marge yelling at Homer in another scene for the angel one, but I haven't seen either episode in so long, I couldn't be sure I wasn't just conflagrating stuff.
This one always stood out to me because Marge's stance on her sister's sexuality was oddly antithetical to her character. She was fine with everyone else being gay, but not Patty (Selma? I always confuse them lol).