However, I think you're wrong about the sex drives of women after 40. A poor sex life is in itself not the cause of a rocky marriage, it is a symptom. If a marriage is good, both partners should be willing to work to meet the other one's needs.
And I would've agreed with you entirely on this point until the last 10 years, when all of our friends, and we, have entered middle age (40+).
I have personal friends and family members who have, on the surface, good marriages that fit the "sexless marriage" definition. Basically, the wife has a lofty college degree and doesn't feel she needs to give in if she is not in the mood. The husband is dedicated to her and the marriage, and tired of being turned down, he stops asking. One husband confided to me that his wife hadn't touched him in a year.
Flangan's article is entitled "The Wifely Duty" and it is meant to provoke and cause howls among women, especially feminists. But it is meant to highlight what these gals did not learn in college or their marriage classes, that men have different sex needs and are actually quite simple. You can spend thousands of dollars in therapy working out issues with anger, frustration, work-home balance, etc., but if the wife just engages in "the act" on a regular basis then all of a sudden good things happen. Roses are delivered to the home on a Thursday. He takes the kids to the park without asking. He actually picks up after himself in the bedroom and makes the bed. He agrees with nary a comment to get the new refrigerator and let her spend money on new clothes. All because she decided ("Flanagan's words") to put out for his sake, not hers. As Flanagan quoted, some of these married gals are giddy with their newfound power over their husbands, and all they had to do was act a little frisky.
Christian organizations have classes about this for women, and my wife went to one with friends. They all came out shaking their heads saying, "I never knew" or "this really bothers me." That's because of messages in college and the media that if she isn't in the mood, then by God he's not going to get anything. Especially if she's been ticked off for months about the garbage, his moodiness, his spending too much time watching ESPN, hanging out with his buddies.
Our grandmothers would have viewed this as common sense. But then again, they weren't filled with education and media that is saturated with messages like "all sex is rape" and "all men are rapists."
So we're in agreement but for the fact that Baby Boomers, at least the younger Boomers, live with the echo of the feminist movement.
Yes, I agree that not all marital problems will be solved by a frisky wife. But if she's wondering why they just don't get along, and she realizes that they have basically a sex-free marriage, she should at least try fixing that part first.