About the ‘times have changed’ narrative. I am a bit older than you, OP (b.1981). There’s the same age gap between you and your ex as me and DH. It’s not about the five years, it’s about the fact you were 13. We met at 25/30. He says in 1994 (at 13 and 18) he’d have seen me as a little girl.
My brother is the same age as DH and I remember very well how he viewed my peers; kids. So he would have been grossed out at messing about with a girl my age. My best mate’s older brother once said to her he had a notion for me when I was about 17. I knew, and I liked him back, but it went unacted on, there was a sense that even then (he was 22) it was a bit off because he had known me as a 13 year old. For context, he had a pal who went out with a girl in our year when they were 16/21 and he got a LOT of slaggings about being a ‘paedo’ even with the age of consent being 16. That was in 1996, and you’re even younger than us.
Re. nowadays, DS is not yet 17 and has said if he was seeing a 13 year old he would fully expect to be a) battered at school and b) arrested. I’ve read this thread to him and he points out that boys his age and younger than him cannot legally ‘look online’ at girls who are 13, so in what world would anyone his age actually think it’s okay, regardless of what the girl is up for? His sister is 13 and he is repulsed at the thought of cracking onto her pals. I said ‘what if they really fancied you though’ and he made a ‘boak’ face. So that seems to be an enduring protective norm.
Leaving the ‘young adult’ grey area behind (ie. 16+ girls) it’s been a good 150 years since a 13 year old was viewed by society as a suitable sexual partner for an adult man, even a young man. Yes, in the 90s there were girls of 13/14 at my school who were shagging men of 18+ but it was seen then as those guys being sickos taking advantage (almost always of vulnerable girls). This isn’t some 2020s sensitivity.
Whether the cops would be interested - arguably not but also possibly not a chapter you’d want to re-open - since OP’s ex has a provable history of targeting young and vulnerable women and isn’t allowed to be around children (WTF) it’s moot. He’s already been caught, just not for what he did to OP. This man clearly didn’t ’grow up’ until some point after his sexual interest in children became prosecutable. And that’s leaving aside an unchallenged history of paedophilic behaviour, coercion, rape, violence and harassment.
As OP says, it’s her son shrugging off what he does know. And expecting her to say ‘it’s no big deal’ to what he doesn’t know but must have a sense of. He’s old enough at 26 to know right from wrong. It concerns me that he believes because it ‘wasn’t violent’ it’s easily minimised. I would argue burglary is an act of extreme violence and violation, and I also wonder if the sexual stuff only ‘wasn’t violent’ because OP complied.
I wonder where her son’s boundaries around consent and exploitation lie. Is it only okay if the perpetrator is popular? Or it’s old news? Why isn’t it up to the victim to decide what’s okay by them? How can he believe that in this day and age?
I think there’s been enough advice on how to handle it, so I won’t add to it, but I just wanted to say to OP that I am sorry and I hope your DS doesn’t have to learn the hard way about his dad. Maybe that’s what it will take. Even if your ex has ‘changed’ (ie. got caught) I don’t see any good argument for you having to suffer through a social situation in the same room with your abuser, and I don’t know how they’ll handle having grandchildren if he’s in their lives. I assume there will be no children at this wedding? He’s storing up a lot of issues for himself and his future family and I suspect his problems are just starting.
You’ve done your best in a terrible situation and unfortunately it seems like for you, as for many abuse victims, your stoicism and loyalty have ended up being used as another tool of victimisation. My heart breaks for you.