Well first of all,,,when he is with you,,,he kisses you,,maybe even a little tongue,,he holds you ever so tightly, tells you he loves you,,talks how his world centers around you,,,how lost he would be with out you,,,He does not do that with her,,with her it is strictly recreational sex,,she isnt getting the deep feelings he has for you. That alone should be just compensation. If you keep that thought,,"well I know he doesnt love her like he does me",,that will help with the jealous feelings,,then kick back and enjoy the ride. You're only going down this path one time,,,enjoy !!