I do think long term relations lead to the problem. I was at a party over the weekend and noticed when the men were in groups together they talked about not having any sex. Meanwhile the wives were flirting with any man that wasn't their husband.
Word to the poster at 15:17.
When I was married, we had less sex ever year until we got divorced (and we were married 24 years.) At first, both our drives were very high but hers dropped off (and dropped like a stone down a well post-menopause.) I am sure I committed a number of the mistakes the women in this sub-forum complain about and these mistakes helped us grow apart in this area.
Every woman that I have dated since says the same thing: she enjoys sex much more now that she is not married or that she is not having it with her XH. I am willing to bet that my XW likes sex more now as well.
Post-divorce, women also seem much more willing to try something new (e.g., sex someplace adventurous) and more open to discussing what they need and what you need. All together, it has helped shift the drive back to high in an amazing way.