If he is using it as a substitute for having sex with you, it's a problem.
If he starts asking you to do stuff that he saw in a porn video and it's not your cup of tea, it's a problem. (If the new stuff is a turn on, then it's all good).
If he starts treating you differently or doing things that are out of character between the sheets, and this new stuff turns you off, it's a problem. (If it turns you on, then it's all good).
If you have asked him to stop and he promised he will, but keeps watching ... then you have a problem.
Best of luck !!