Our marriage is fine in every other aspect, except the sex.

Not to be a wet blanket, but I just cannot believe that all of your "extra sex" has not had an adverse effect on his sexual interest. As one who got caught in "extra sex" a long time ago, I can tell you that it is humiliating to the "victim spouse." It is hard to believe he does not care. I think this is the root problem and you should deal with that aspect of it.