A pretty sensitive subject that is...

Honestly, from a nature point of view and from the view of our urges,
it appears obvious that men have more needs for sex than most women.

Maybe he's not in the mood to have sex and please you as he only would like
to release some pressue, knowing that if he'd do so with you, he'd likely dissapoint you and you'd feel used.
If it's not frequent, he likely just didn't want to bother you for that. Watching a porno to it, just speeds things up and he might be more open for love and tenderness again, all just for you.

If he's not watching them, he could well be thinking of others while wanking, couldn't he? And for a fact, that's neither controllable nor bad in my humble opinion.

It doesn't have to be that, but it's one of the possible answers though. We _are_ different after all, and we very much feel different in especially that respect, mostly.

If it doesn't happen all week, I wouldn't worry.

Just my 0.02$

Cheers,
Carisma