At this point I'm going to have to agree with what Toaster said about gender...
If it seems like he's saying sex takes too much effort, next time you just absolutely can't contain yourself wanting to have sex with him try and set something up so that all he has to do is sit back and enjoy and not have to worry about if he's doing this or that or anything at all that would take his mind off of how great it is to have sex with you and how awesome it feels. Maybe sex has just become such a big issue in his mind (that he can't satisfy you or isn't good enough) that he has a hard time really getting into it without thinking about all of those things. Make sure he knows that HE is what you're into and that the sex part is really just a wonderful side benefit of your marriage.
I'd be really surprised if he didn't come around eventually, just try your hardest not to ever be negative about it or critical of him. Sex is a great and wonderful experience for you two to share and with four kids he's obviously been into it at some point.
