Been there! Bit I couldn't get at the time, is that she totally enjoyed the physical contact, the massages, kissing, discussion, foreplay, arousal and petting etc, came several times before, during and occasionally after, chat and cool down (I'm not a roll over and sleep guy) - but would she every initiate? no. would she ever make time for us to enough it and have her fuss over me? NO.
She said she just considered it was something she was expected to do as a good wife and yes it was great, no she never faked*. She just really couldn't bothered most of the time, and I never seemed properly thankful for her efforts (ie during the rest of the week, I just kept bugging her for more). There were no kids in relationship, that happened right at the end when we went on holiday, so I was given "bonus sex" a couple of nights that week while we were away.
I pointed out it was an intimacy thing and really importantly to relationships and me (very high and wide sex drive - that she loved when we originally dated). She said yes she knows that, which is why she usually agreed to "it" once a month if she felt in the mood "for it", and I was "getting an unthankful attitude again" and she had been a good wife and done the sex for the marriage, but was pretty much over it all and wanted to move on. ...
"Intimacy" and "sex" had just been things she did, like washing or picking up milk. And when she wasn't "buying in"/investing into "a marriage" then the transaction side came out - what was *I* paying her, for sex. It was then I realised while I was emotional inside a "man-team" where everyone is in and shoulder to shoulder helping each other out with fraternal bonds; for many women a relationship is just another transaction, a social contract, if she does X then she is entitled to Y, and that the contract required certain trades (in her mind, sex for house - not for fidelity that was presumed**), and was only valid for as long as she wanted it/was benefiting from it.
** nothing says entitled like listening to a woman complaining about her husband having an possible affair, to her fu..toy.
*this is well after the divorce, and also feedback overheard via her friends, so consider it mostly true