I don't know whether to say 'You don't need to tell me about this particular guy, but let's stick to the old way about new people', or what? I'd find it really, really useful to hear your specific experience on this and what works best for you.
What do you get from that in the long run, you'll assume she slept with him but not have verbal confirmation? Anytime she starts dating somebody you don't like do you ask for this again? I can see not wanting to discuss it in length, but I'm curious why a statement is has happened is more stressful than the assumption it's happened. Can you figure out what feels better about not discussing it at all? Is it just that you don't want to hear the process as she goes through sharing where she is at on the road to sex with him?
There's nothing wrong with saying "I dont really feel good about this person, do what you want but I don't want to hear about it" But I think its good to make sure you know what its about - maybe you have valid reasons for thinking they are not a good partner and so this is just an exception, maybe there are some personal issues to work on. I like knowing when other relationships become sexual but to me that's just a status update - dating -sexual relationship -moved in together - committed but not planning on merging households- engaged, etc.
How does you knowing she had sex with him hamper her sexual enjoyment? You mention worrying about her sexual freedom and spontaneity, could a bit of this be you projecting and wanting to have your own spontaneity but find it easier to couch it terms about what she might want?
You mention worry about hurting each other or breaking guidelines. If it's what you want, is it possible to revisit agreements so you're both comfortable with spontaneous sex being fine happening as long as safe sex rules are followed and it's brought up quickly afterwards?
Just to note - my personal experience, my husband and I wouldn't likely have spontaneous new sex partners, if we did for some reason, safe sex rules should be followed and it'd be brought up ASAP. To be clear neither of say we are comfortable with a do whatever then let me know afterwards arrangement. We share a computer room so chat about "I like this person" I'm messaging this person" I want to ask this person out for coffee" is natural. I'd find it untenable to have Adam tell me he didn't want to what was happening with somebody, I just wouldn't date them then if he had that response to them when he'd been fine with everybody else I'd dated. As I couldn't avoid knowing when he was on a date with somebody even if I didn't like them, I'd expect to know when they had sex so that conversations like "the condom broke last night, her lasts STI tests were in September and were negative, but I am going to do ___ about it/Pregnancy worry discussion needs to happen" or "Soandso's SO tested positive for ____" so what do we do about this" or "I want to spend the night at Soandso's house" would make sense and be in perspective.
My boyfriend, I'd prefer to know if starts dating somebody and is thinking of having sex with them, but if things moved fast I'd at least expect to find out before we had sex again (I'd tell him the same) Besides knowing about new partners that'd really be the extent of it. As I don't think he'd welcome more discussion than that, if I liked the person or not he was seeing it'd really be a moot point. No clue if any of that babble was helpful.