I had this, it started with porn watching, then escalated to webcams, he once spent 100 quid in about 10 minutes on them, by the time I found out he was already visiting brothels, all done on way to and from work and some lunch times. All this was revealed when I discovered he was cheating with a woman from his work. He only ever admitted to what I had evidence of, I always had to present proof.
In my experience they don't stop, only escalates in use and they become better at hiding what they are at. It must be like chasing a high. As for the therapy I tried that too, 70 quid an hour I paid for therapy privately, and he just lied through that and blamed me, as we were "working through things" he was still messaging escorts and arranging times and places to meet, he swore up and down that he never followed through but who knows. I wish I had that time back again, I wasted three years trying to "work through things" I ended up on anti anxiety meds, sleeping tablets and anti-depressants and all the time the therapist was making out I just had to let things go, to forgive and move on for things to work out. Whatever about the heartbreak, whatever about the waste of money, the thing I most regret is the wasted opportunity. In those three years I could have moved on, healed and started rebuilding my life sooner. I will always regret that wasted opportunity.
What people don't get about this behaviour is that it is part of a larger pattern of abuse, for a person to be so consumed by this behaviour they must reject their partner on an ongoing daily basis, in my case nothing was ever good enough, he would turn his head away if I tried to kiss him goodbye in the mornings, he would make little jokes and sly digs about me in front of others, it was always low level little things, that individually you may not even notice, and are always such little things so why get sad or mad or worked up by it, but in the larger picture your self esteem crumbles, your confidence goes, you doubt every action and convince yourself that you are sensitive or touchy or whatever word you want to use to make your partner's treatment of you OK. In the end you are doubting if you are worth anything and convince yourself that you are not worth more, but the thing that always hurts is the loss of opportunity.