OP, you asked a while back about the general concept of forgiveness and second chances. That got shut down a bit, but I do think it's an important point.
I think I come to this a bit differently because while being very anti-prostitution (and anti-porn, etc.) I also have known a lot a lot of people in my life who have done very bad things in their past. (Thanks to my dysfunctional upbringing and years spent in dodgy scenes.)
I personally think, especially if we are talking 20+ years ago, that the act is important but even more important is whether they have sorry to use a religious term repented of that act. Not just that they are embarrassed, but that they have really repented.
So for example, I have known some people who were involved in gangs when younger, who turned their lives around and now do a lot in their community to keep kids out of gangs.
It sounds like your boyfriend is ashamed and remorseful, but if it were me, I would want to see real repentence. For me, this would mean: A) a real understanding of why it was wrong, not just from his perspective but from the perspective of women; and B) a real determination to never again treat women misogynistically or badly. Ideally it would also include C) a yearly donation to a charity that helps women escape prostitution.
I just thought this might help in terms of framing your discussion with him. It's not really about whether you can live with his mistake, it's about whether you are comfortable with his emotional and moral reaction to committing a wrongful act. It's about whether you are living in the same ethical sphere.
If you are not convinced, let him go -- not to punish him, but because it would just mean you are not morally compatible.