Tris Harkness has written about this extensively - how she always felt coerced and that sex was demanded as part of her "wifely" duties. When they opened up their relationship - ostensibly to let him have what he wanted elsewhere, her libido came alive because the pressure was off. She got to experience her sexuality on her own terms and it made things so much hotter, both with new male interests but with her husband as well. I don't think all couples need to open up their relationship, but understanding that this sort of coercion may well be the root of the problem for many women would be beneficial to so many relationships.