Nope I would never do this to my mum, and I walked on on my parents a couple of times in my youth (woken in the night and gone down to my parents room for consolation and found myself an unwelcome intrusion😬) my parents also divorced in the early 80s.
The 80s were a terrible time for divorce and very tough on children then. There was a record surge in divorces, but it was still a lot less ubiquitous than now. I was only one of two children in my class with divorced parents. That made me feel set apart and ashamed. There was a lot less shielding of the children and emphasis on friendly co-parenting than now.
I think children of that time were a different generation. We were still raised with that 70s independence so we're really self aware and curious, especially if bored. I remember I would hunt through stuff to find interesting things. Objectively to a child a dildo is a riske object of fascination. I used to go through my mother's knickers drawer, because I thought all the silky fabrics, lace and colours were really beautiful. I found knickers that made no sense to me at the time, because the crotch area was missing, or it just had a string. I thought they must go over my mum's arms somehow😅 It's par the course of living with adults in a confined space. But I can well imagine being totally confused if I had seen my parents having sex when they weren't getting on and interpreting it as things are going to be alright. Only to be crushed when it's not alright. Then irrationally blaming my mother for accepting sex under those circumstances. Feeling inappropriately deceived.
Obviously as an adult, I now know sex and emotions are complicated. Some people continue having sex up to and even after divorce. Even sometimes even when there's another marriage by one half of the divorced couple, unbelievably. So there's no judgement to be had. You can screw your husband right up to signing the papers. It's your perogative.
I therefore think your daughter displayed some intellectual immaturity, (odd for someone 30 yes old) as well as resentment. The resentment might be that (like myself) she wasn't wholly and adequately protected from the fallout of your divorce or it was never explained in a way that made sense. I don't know, I'm guessing. It seems she enjoyed embarrassing you in that moment. The question is not how could she embarrass me and not care? The question is what is underlying that response. Why has she held onto it? That's what you need to figure out.
I do think not apologising is deliberate on her part. It would upset me too. I'd want to discuss it.
But also:. You know our children, we love them, but they're not perfect and can have some bad traits too that we do not like. You may just have to accept that she has the capacity to be spiteful and you're never going to get an apology, but that doesn't mean she doesn't still love you dearly. One of mine never really apologises. The others do immediately if they sense hurt. The one that never apologises, however, is the first to offer help if I need it. Swings and roundabouts.
Weigh everything up and try to just get past it and forgive, if she's not willing to discuss it further.