I felt I should chime in here.
I'm male and I often go to this website as a substitute for porn. I have never once met any of the girls off this site, but there is something kinda kinky about knowing that you could have sex with these women if you were really willing to.
I don't think I am alone here because one time I was fixing one of my room mates laptops and happened to see his history and found the same thing and I know for a fact that he never snook out to sleep with prostitutes.
Now, I am not saying this is the case with your husband, but it does seem plausible. The only way to truly know is to check his phone records and see if he has called any of these women. If you are going to be with a prostitute you would have to make a few calls to arrange and directions etc, so I would imagine that if you saw single short call to one of them, he might have been ringing to see what they sound like.
I hope for your sake he isn't doing this, but the only reason I am posting here is to let you know that his story is plausible.