Something I think about a lot is the difference between the ways men and women learn about sex. I don't know if this is just me, but I don't think it is, and I do think it's relevant.
At the risk of generalising or opening a can of worms, I suspect a lot of women's early sexual experiences are not exceptionally gratifying. I know mine weren't - it took a long time for me to know my body well enough and be confident enough to tell a partner what was needed in order for me to get off. I just feel like there's a huge amount of false information, from media, even books, just everywhere, about what good sex for a woman consists of. As a result, I spent much of my early sexual years... not pretending, per se, but there was definitely a certain amount of generosity in my responses to sex, in that I felt like I 'should' be enjoying it - and I did! But not always as much as I was indicating. Basically, I learned quite young to - I suppose compromise, sexually, in that I wanted my partners to enjoy themselves, and I got used to the idea that they would almost always orgasm, and I might not.
What I'm getting at, is that I think a lot of women are quite used to sexual compromise, and I don't think men are, to the same extent. And I do think that when you get to an issue like this, those years of learning and socialisation make it difficult.
I don't know, maybe I'm just really wordily saying - hey, I've had sex loads of times when I didn't really feel like it, and I don't think my partners were ever aware that they were getting, for want of a better word 'pity' or 'obligatory' sex. And I wouldn't have wanted them to - because for me, it's part of making someone happy and in a loving relationship we all do lots of things like that. But in my experience men don't seem to be able to do this as easily.