It´s hard for me to make any guesses but I would say that the changes in his sexual responses (which you have described) do not have to be related to cheating. Of course, I have no evidence that he is not cheating, but I would not think this is the case only because of the sexual changes which you described. Unless you have any further evidence or at least signs indicating the probability of his cheating, I would try not to worry too much about it as it is pretty possible that he is "innocent" in this.
Yes, you are right, many married men cheat, but our mind tends to notice mainly these negative or threatening examples (in strip clubs, among friends etc) and we often do not focus our attention to the faithful couples and faithful men.
As for the decrease in his sexual desire, there may be many causes, e.g. calmer stage of the relationship (not as wild as during the fascination stage at the beginning), routine, psychological factors in the relationship or outside the relationship (job - psychological stress can be even more dangerous to libido than physical exhaustion) etc...
If you can think of anything which you haven´t tried together and which he might find sexually arousing (sex toys, role playing, watching porn together, different kinds of stimulation...), you can try to introduce it into your sexual life and it might hopefully go wild again...
First things first, I'm a 20-year-old unmarried male student, so obviously when it comes to advanced relationship (read: marriage) advice, I'm not an expert. However, I myself am a guy, and I can give advice based upon my common knowledge of the male psyche and relatively educated guesses, but at the same time these are no more than just that: guesses.
Before going further, it is perfectly possible that it is a medical condition of sorts, or some other physical factor, and this should definitely not be ruled out. Perhaps it has been caused by a change in diet, lack (or increase?) in physical exercise, perhaps he does something else now. Additionally it could just be caused by the fact that you're not "new" anymore, and instead of him automatically thinking of you when he's at work (which could maybe lead to him being "full", as he's been working himself [no pun intended] up during work and can't wait until he can see you), but now he doesn't do it automatically. You could maybe try with doing something that will arouse him throughout the day as sexual frustration can lead to us men become quite primeval, and if you hint and flirt with him throughout the day, but during circumstances where there will be no opportunity to have sex, such as during lunch on a workday. (Disclaimer: just examples, just do it however you like and feels natural to you)
As you already said, he could also be cheating or just be a chronic (and ashamed) masturbater. You said he's with you always when he's home, and that could also be an indication of such, especially if he's also nice, as he may be feeling bad and want to 'spoil' you when he gets back. You cannot confront him directly, as this confrontation will lead to hurt for both you and him, regardless if he has cheated or not. Little things like, noticing if he takes his cell phone with him to the bathroom or is secretive about some stuff can give some indications, but no proof, and frankly, someone else will have to provide more in-depth advice on figuring this one out.
Hope my insights/opinions have helped.