Some brilliant answers on here;
The fault is with him- not just for being unreasonable and an arse and trying to pressurise you- what a horrible thing to do- but mainly - if you are not in the mood for sex, could he MAYBE not be wooing you sufficiently?!
I mean, the old saying of men being like kettles (heat up in 1 minute and then it's all over) and women being like irons (ages to heat up and cool down) is very true, but it seems he has no idea how irons work.
Also what about "you have to heat up the oven before you stick in the turkey"?! He is never going to understand this unless you make it clear. Women NEED to be put in the right mood for sex, otherwise IT DOESN'T WORK! Physically, it does not work. Does he do any foreplay? Does he make an effort to make you feel special and desirable? I'm afraid that it necessary. We can't just drop 'em like men can. We are programmed completely differently.
Get him to watch some ducks. This time of year they go around in pairs, the male following the female. She constantly ignores him and he constantly follows. The males fight over one female. Eventually after much wooing, she lets him jump on her. Conclusion: Men are programmed to need sex as often as possible in order to continue the human race. For some reason, women aren't!! What would happen if they were? A population explosion for one thing.
Go to sex therpay and they will tell him in no uncertain terms that he is in the wrong here.
Or just dump him and find someone nicer, more appreciative and understanding.