They met on a cougar-dating site because he was there looking for sex with an older woman (ie, to fulfill a fantasy) and she was there looking for sex with a younger man (ie, to fulfill a fantasy). They met twice, had sex, he's said from the start he's not entirely comfortable with this, feels he's in too deep, expressed reservations, and she's not clear what's going on?
He had sex with her, it was fun, but he's apparently 1) not that into her and/or 2) not that into the idea of being the boytoy with no hope for a real relationship. It could be either, it could be both. This isn't terribly hard to understand. There are some real major drawbacks to dating a married person. There are generally some limitations to how far such a relationship can progress.
They've met twice? A 5 hour meeting and a 6 hour meeting? And the second meeting ended with a half hour confrontation? And she hasn't seen him or heard much from him in something like 4 weeks and what contact she has had has been 'strained?'
Do you and A realize that this means you're talking about a 'relationship' that can literally be measured in hours? And if we throw in the 4 weeks, that makes something like 95% of this 'relationship' strained.
I'm trying to put myself in S's shoes: I see a guy twice, for a grand total of 11 hours, and he now spends 4 weeks telling me he wants to 'talk,' wants me to 'reconsider,' wants to know 'what went wrong,' telling me he 'needs answers.'
I'd be seriously considering getting a restraining order.
Honestly, I think the answer is, this guy had a cougar fantasy, he fulfilled it, it was fun, he kind of liked her, but not enough to continue, he doesn't want to be involved with a married woman, and now he'd like to be left alone.
The answer is: he thinks they should call it a day because they're playing with fire.
Maybe it's time she backs off and leaves him alone. If he's that into her, he's got her number.