My partner viewing or downloading porn, wouldn't really bother me, as I think most men and some women are into it.
However, the fact he is registering on sites to actually meet other women for sex, is really not on. Also, as he see's nothing wrong with this tells me, he really doesn't give a damn what you think or that he is causing you pain and upset.
I think you should do the right thing, kick this sleaze bag into touch, so he can go off and meet as many 18-55 year olds as he likes. You deserve someone who will respect and treat you with decency and regard.