Yes, I do think all prostitution is wrong. Some forms more wrong than others. The intrinsic and unavoidable power dynamics involved with the Thai sex trade make it almost as wrong as its possible to get, IMO.
I also think that the sexual exploitation of women is so ingrained in our culture, and the domination of women so inscribed within our sexuality, that it is quite possible and common for men who are apparently decent, gentle etc, to abuse women and convince themselves that they are not doing so.
So I don't think your dilemma is that unusual, HJ. And I do feel sorry for you: it is really horrible when you find out that someone you love is capable of not doing terrible things. I've been there myself. And, with the wider society forgiving and condoning what he's done, it must ffeel very po-faced/punitive/pointless to punish him for what he's done. You must be thinking: he's a good guy generally, I can do more good by staying friends with him and gently persuading, than just cutting him out of my life.
And that may be right. But personally, I couldn't do it. Not just because staying friends with a man who had done this would make a complete mockery of everything I have believed in for over quarter of a century. But, more pertinently, because I would never be able to trust him, look at him in the same way, or forget what had happened. It would poison our friendship forever.