Was I in the wrong or was DSis being a bridezilla?

Why do you think it’s OK to police what other people say? It’s not your wedding and other people can say whatever they like. You can disapprove, but you have no right to tell anyone what they can and can’t talk about. And older relatives don’t need you to sanitise the conversation for them.

You are not in charge of deciding or policing what's appropriate though.

I didn't missed the point at all, I am not a total killjoy but if someone is so drunk that they are unaware of surroundings and not able to" read the room" then imo a gentle nudge might be helpful for everyone's comfort. The op didn't say it at the table or make a thing of it, the bridesmaid was irritated and passed it on to the bride, who went in heavy handed to her sister about it, and made it a huge deal.

You were being unreasonable to police one adult's behaviour on behalf of other adults whose opinions and agency you had no right to take charge of. If you were actually uncomfortable yourself with hearing the explicit conversation the adult thing to do would be to start a different conversation with whoever was seated in the opposite direction than the person you didn't want to hear. If the actual problem was your assumptions about the delicate sensibilities of other guests then it's just totally patronising to take it on yourself to assume that - anyone who was upset has their own agency and choice about what to do about it. This is nowhere near a bridezilla situation and not particularly to do with it being a wedding, just a woman finding her over-opinionated and busybody sister just too tiresome to tolerate any more.

I think it depends on how you suggested politely that she reign it in.

I wouldn't have told her off as such eg, …you really shouldn't go round telling your sexual history to strangers at somebody else’s wedding !

But I might have said how I felt eg…. Crikey Jane, I felt so embarrassed and uncomfortable with you broadcasting that sex story in detail to the table where my parents and grandparents and elderly aunts could hear every word.

Either way, the bridesmaid was the one out of order, she could and should have kept your conversation with her to herself instead of telling the bride. It was bad enough that she relayed the story at the table in the first place.

Your sister’s memory of her wedding was surely tainted by this. She is feeling defensive and so lashed out at you. If you hadn’t spoken up everybidy could have pretended it didn’t happen and her wedding memories would still be pure and lovely.
Let’s hope she calms down and everybody can quickly forget it and move on.

At their grandmother’s funeral wake, my partner’s nieces and nephews (all late teens /early 20s) were talking in very sexually explicit terms about some pornography they’d seen or heard about. I was the only adult who could hear them and was appalled. I didn’t say anything, but with hindsight I think I should have. It was really not the time or place and they should have been told that in no uncertain terms. I was a coward that day, but the OP @Weddingdrama26 wasn’t , so good for her.

You cannot seriously compare the funeral of a grand-mother with the wedding of your best friend.

And there are ways to go about it.

I think a better thing to do would have been to walk away and just let the bridesmaid embarrass herself. No need for you to take it upon yourself to get involved!

Unless there is some major backstory or the OP has omitted some important details about the bathroom intervention ... YANBU.

This is not a matter of protecting the elderly in particular. The bridesmaid was forcing (whether intentionally or otherwise) a bunch of people in the vicinity to participate in a fairly graphic conversation about her sex life. It's fair enough to say something; and in fact, I think the OP did so in way that was discreet and minimised any conflict or public embarrassment.

If this was in front of children then absolutely something should have been said, in front of adults then they have tongues in their heads and if they found the topics unpleasant they should say something.

I'm quite surprised at the vote result. Yes, it sounds like her tales were pretty rough for the context, but there were no children about, right? Those whose sensibilities were offended were free to speak up on their own behalf in the moment, or - better yet - simply ignore the offending talk. Based on your description of this scenario, you sound like a busybody.

Your sister has overreacted, however. Do you perhaps have form for trying to control others, and she feels you have interfered with festivities by following a pattern that she is fed up with? If so, maybe acknowledge this and apologise. If not, let's hope it blows over soon.

Why is it such a big deal???

I bet the older relatives were more annoyed you interfered & ruined their 'earwigging gossip fest' & them not finding out the ending to it all. Seriously through, I don't think you did anything wrong wanting to keep your DSis wedding respectful.

More patronising BS about "oldies". Wait till you're old and see how interested you are in some silly girl's sordid sex stories. I guess younger generations will always assume they invented sex.