So if he is turning down sex with you, it must mean the sex you are providing is not the kind of sex he wants to be having.
What I mean is... what is he into ? Maybe try giving him head and not leading to sex, just finish him off with a bj. Maybe he likes oral sex more than regular sex ? What kind of porn does he watch most often ? That will tell you what he is into the most.
Men don't marry just because, in fact, most guys are reluctant to get married to anyone, so if he married you, something about you must have turned him on and he must have loved you. All you have to do is give him what he likes sexually, talk about what he would like in bed, if he doesn't want to talk about it, then maybe rub Hiscock at night and whisper in his ear what he'd like you to do, this way, while he is horny, he is more likely to open up about what he wants from you that he won't mention during daylight hours.
This is affecting your life so it's worth fixing. Best of luck !