Wow! It could be many differnt things...his sex drive could be down, he could be on some type of meds, he could be having an affair. I would suggest you sit down and ask him what is goiing on. Try to find out before thinking the worse..
Talk to him. Something is seriously wrong. Maybe he is concerned about pregnancy? Maybe he has a girlfriend and doesn't want to cheat on her? Maybe he is homosexual? Maybe he has an STD or AIDS and doesn't want to give it to you?<br />
There is something very very wrong. You need to confront him on this.
ask him, seek a sexuall doctor and sex surrogate for advice, have sex with other guys or gals and tell him about it, let him watch you **********, maybe get him excited, keep trying, do not give up