If he is signed up for an online adult website, then he might be cheating. His cheating might indicate a problem with your relationship (between the two of you). What can he get from the website that he doesn't get from you? Does he get enough sex from you? Does he want to try new things you don't want to try? There's your answer. He really wants certain things but is not getting them from you.
Or he may simply be a slimy person who doesn't really care about you.
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)