IMO, this is a guilt issue (possibly even instigated by some passive-aggressive behavior from you)...
It's entirely likely he's imagining other women, many women at once, wild situations, etc. during LM. But..frankly, this isn't unusual in men. Men are stimulated visually.
Where you're getting that "there's something missing" feeling is, I believe you're picking up his subconscious guilt in having those fantasies because he feels you would "disapprove" or would feel jealous (true or not).
A suggestion:
Tell him you'd like to watch the adult films WITH him. This will let him know you're OK with it. Even BETTER, bury your inhibitions and attempt to let it stoke YOUR libido and go ahead and play (but not out of obligation - he'll KNOW). You'll find it won't take long before YOU become his primary supplier of pleasure, even in his fantasy world. As that happens, the porno watching will become more rare along with that "something's missing" feeling after LM.
