I have confronted him about all this and he tells me he is sorry, embarrassed and he doesn't know why he did it and has promised not to do it again. He tells me he loves me and wants to put this behind us and move on.
Oh for goodness sake. He knows exactly why he did this. He's looking for something more, as Haxan has already said. He's sent racy messages and his nude photo to other girls. This is about *him*, not you.
This is not a guy who is into you, and its pretty close to what I'd call cheating. Go find a nice man who will respect you. There are plenty of good guys who are amazing in bed and will inspire you to be amazing too. This one is already making you insecure and it will only get worse. Get rid of him already.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh