I've come across this word often on PPD and even in mainstream discussions of sex and relationships.

And there was always something about the verbiage that discomfited me.

Essentially "withhold" is the incorrect term.

A woman (or man) does not "withhold" sex from their partner. That woman is simply not horny and not inspired to have sex. She doesn't desire sex. There is no lust in her at that moment. And if we know anything about sex humans only want to have sex when they are lusty.

For men they start getting an erection... For women they get wetter, genital area starts throbbing, and/or breathing quickens.

A man (or woman) does not "withhold" attention (affection) from their partner. That man is simply not inspired to give it. He doesn't have warm or amorous feelings towards his partner at that moment. And if we know anything about the humans we know they only desire to give attention when they feel a desire to be around or are enthralled with a person.

Now of course I believe in you decided to commit to someone for the long-haul, if you want the relationship to be mutually rewarding and satisfying and loving, then the person who "isn't inspired" needs to figure out why and what would make them start desiring to fuck and / or be around their partner more.

But I also think the partner who isn't getting what they want should have the presence of mind to know it's not "withholding." Their partner isn't withholding. Their partner doesn't want to because they don't feel sexually or lovey dovey towards you. Troubleshoot that first. And the first action step toward troubleshooting is to stop mislabeling the issue. It's willful ignorance/denial to do so.