Boyfriend signed up for several on line "xrated" singles sites & other sites not x-rated. he?

Need some help..Boyfriend signed up for several "x-rated" single/dating sites. He gets regular messages from other members from searches he did within our local area. Requesting to meet/hook up for sex & of course racy pics. I feel as if it should b enough that he spends a lot of time on internet & phone when laptop not available looking at porn, vidoes,live webcams. Why put yourself available on those sites when he is in a live-in relationship? Its like he's wih me but still looking for something better. I've asked him to stop, then to find he's signed up for couple more and lying abou it. He says he's never answered but if he's lied about beibg member how can I believe that. Am I making too big a fuss over this?? Should I just shut up and let him contnue hoping it's really harmless and he's really just looking??