Yea, it's a dbag move to say that type of thing to your wife but if that's the worst thing that's happened in their marriage of 20+ years, I think they are doing pretty good. So we're going to pin the lack of sex now on one offhanded, stupid comment? Then she was looking for an excuse anyway. It could have been any other innocuous comment or disagreement over anything that she would use to drop her interest in sex. I used to think this type of stuff was random but I've learned it's not. You don't wake up one day and she's suddenly so upset over one comment that she's not interested in sex at all anymore. So she has no physical desires then? No, this was planned. Perhaps she got it in her head from one of her friends that she could get better on the market. Perhaps some other guy is showing her attention. Perhaps she got this silly idea all on her own (not likely but possible) but this was planned. She's wanting to get out there again and this is her way of forcing him out the door, plain and simple.
This is where men and women differ if guess, because a woman, I can tell you with 100% certainty, would never consider a comment like that off hand, nor would she forget it even if he said it only once.
One negative comment like that is all it takes to dry her up, for him anyway.
But I’m sure there are other things going on like you suggest, and not just this one thing. OP did mention that she took long to orgasm so I’m imagine there is some sexual incompatibility as well, coupled a lack of communication perhaps?
It’s not uncommon for a woman, believed to be frigid in the area of sex, to ‘magically’ awaken ‘down there’ with the right guy bringing the right kind of masculine sexuality that draws it out of her.
This is why when she says she’s done with sex as OP stated in an earlier post, as a woman, I’m calling BS on her because I know that it’s more likely that she just doesn’t want to have sex with him, probably realizes it’s selfish to deny him that (which it is), and is offering him a hall pass.
If they both commiticated better with each other about what was lacking and what they each needed to have a better sex life perhaps it would not have come to this point. Or maybe they have communicated it and this is the point they have come to. That’s sad on her end that she doesn’t seem to want to try but again, she may have her own agenda around that.