1. Women's sexuality does not function like male sexuality. Women's sexuality tends to be a lot more fluid then men's sexuality. Just as an example, you never hear a straight man all the sudden in his 40s come out as gay, unless he has been closeted all his life. Or conversely, you never hear of a gay man all of the sudden becoming straight. Yet, we hear a lot of stories about women who have lived happily and honestly heterosexual lives, all the sudden later in life becoming predominantly attracted to women. You also hear a lot of stories of lesbian who, when they get to the age when their biological clock turns into Big Ben, they really want to fuck men. Also, bisexuality is more common in women than in men. This same fluidity can be applied to sexual appetites. If a man, for example, is into being submissive, chances are that he has felt that way since his formative sexual years, and he's going to feel that way forever. That's not to say he isn't capable of exploring and enjoying things outside of that fantasy...but chances are, that's always going to be the #1 thing that gets his juices flowing. This is not usually the case with women. True, many of us do have favorite fantasies, but they tend to change more over time, and what turned us on 10 years ago may not turn us on today.
2. Piggybacking off of that, women's hormone levels change a lot more than men's do over the course of their lifetime. Men's sex drive, though it peaks typically in their late teens/early twenties, is pretty fixed. That's not how a woman's sex drive works (that's why a lot of ladies in the biz take extended breaks!). Sometimes, it's go go go! Other times, it's non existent, and it's just how our sex drives ebb and flow.
3. Women are brought up in a world where we are taught to please men, and porn especially has upped the ante over the past 20 years about what is and isn't "normal" and what is to be expected. Most of this, of course, falls on women's shoulders (how many guys are expected to bend and contort themselves into extremely uncomfortable positions just to get their lady off?). Furthermore, for many, porn is the only avenue to learn about sex for pleasure, and women internalize what they see. So many will do these things that they're not into, and that they've never been into, just to please their man and keep them by their side. And that's really, really difficult for most women to sustain for a long period of time. Eventually, they take a posture of "been there, done that. I gave you what you wanted, so how about you give me what I want by just leaving me the fuck alone?" Sad, but true.
4. It is more physically and emotionally taxing to be penetrated than to penetrate. And again, if that is constantly part of the routine, and always to be expected, some women do crash and burn. And it even happens to some women who are totally into it, and get off that way. Women do tend to be more emotional creatures than men, and so you can really overload that circuit if you're not careful. Some women do find that it takes the pressure off them when vaginal/anal sex are not always on the menu, and there are sessions mixed in that are "just" oral and/or mutual masturbation.
I hope this helps give you a little bit more insight as to why these things happen. I know that it is frustrating when all of the sudden the sex dries up, and there seems to be no interest in getting it back. But perhaps this bit of insight will at least help you sympathize and understand the fairer sex's viewpoint.
