This is certainly a possibility but I don't think it's the main reason that sex tends to drop off in established relationships. I honestly don't think most people are that calculating. Women get bored quickly with the same old same old sex, which kills their libido. This is something that's been studied quite a bit.
https://medium.com/the-atlantic/the-bored-sex-3d046dc562c
Although most people in sexual partnerships end up facing the conundrum biologists call “habituation to a stimulus” over time, a growing body of research suggests that heterosexual women, in the aggregate, are likely to face this problem earlier in the relationship than men. And that disparity tends not to even out over time. In general, men can manage wanting what they already have, while women struggle with it.
“Long-term relationships are tough on desire, and particularly on female desire,” she said.
But Meana discovered that “institutionalization of the relationship, overfamiliarity, and desexualization of roles” in a long-term heterosexual partnership mess with female passion especially — a conclusion that’s consistent with other recent studies.
“Moving In With Your Boyfriend Can Kill Your Sex Drive” was how Newsweek distilled a 2017 study of more than 11,500 British adults aged 16 to 74. It found that for “women only, lack of interest in sex was higher among those in a relationship of over one year in duration,” and that “women living with a partner were more likely to lack interest in sex than those in other relationship categories.”
Are there "love bombers" and gold diggers out there? Sure there are, but because we have a cultural narrative that what women really want is to nest with a partner and have emotional closeness and security, we fail to recognize that in many, many cases women in established relationships are not getting their sexual needs met and are bored with the sex available to them, and so they kind of go off sex. Female sexuality is often predicated on feeling desired, so if after a while she's just sort of treated like the roommate who does the cooking and the laundry, and the person you are "allowed" to satisfy your sexually urges with, it puts a real damper on her libido.
"When men complain that their wives don’t respond to their amorous advances, chances are they are approaching them as husbands rather than as admirers. They make the mistake of thinking that a wife wants primarily to be loved, when in fact she principally wants to be desired, not just loved and taken care of. A husband who approaches a woman without wooing her is not likely to get much of a response, because he hasn’t addressed her core need." ~Rabbi Schmuley Boteach
As you've said, there are other possibilities and sometimes men go off sex too, but this info about women and desire just hasn't quite made it into the collective consciousness yet.
