Cally
I am really really sorry. I went through that exact same thing six years ago when I found a subscription to one of these services. I was furious and he explained he was simply curious. Stupidly I believed. A year later he met a couple of those women and I found out. He said no affair, just meeting. We worked on our marital issues and I thought we had built a stronger marriage. Five year fast forward and suddenly I have discovered that for nearly 10 years he has been visiting prostitutes. A lot of our issues stem from his unhappiness with his life in general but some also come from issues in our relationship.

If I could redo my life, I would be far more insistent on figuring out what was going on the first time I saw that he had logged onto these websites. This does mean the end of your marriage but it certainly is a wakeup call.
I apologise for projecting from my own experience... but I do remember the pain and feel really bad for you.
DeepBlue