@handholdplease9 I am so very sorry OP, and I understand your revulsion completely, but you are also in total shock at the moment, and until you know for absolute certain, when you are no longer in such a state of shock, could you put off making such a final decision for a while? Obviously you can't live normally with him in the meantime, is there anyone else that he could stay with for a while - a good friend of his, or a sibling maybe?
I understand your very kind reason for not wanting to upset his Mum, you are obviously a lovely person, and as others have said, none of his issues are your fault.
I will probably have every other Mumsnetter on here shouting at me and being rude, but I am only trying to make sure that you don't do now, what it is possible you could regret for years. I have to admit that although I have not got any sexual deviant desires myself, I have no problem with other people behaving in any sexual way they want as long as their behaviour is both legal, and consensual if it involves other participants. Therefore, I would not be suffering in the same way as you are - even though your reaction is entirely normal and understandable in the circumstances, whereas mine probably isn't! What I would be upset about, is him not telling me the truth about his past behaviour and his depression - I think that at least the depression part of his explanation probably is true, and he would have been very scared of losing you if he admitted it to you, especially if his past behaviour is no longer relevant in his life.
If this was my situation I would tell him that if there were to be any chance for our relationship surviving this massive shock, I would need to know everything, including the truth about what appears to be quite recent additions to this site, and if he admitted it was him, why did he still feel the need to be a part of it, now that he had your love and devotion? If we were to have a future, I think that he would need to see a counsellor who specialised in this sort of sexual behaviour, and when he had gained some sort of understanding from that, he would need to be willing to go to joint partner counselling with you, and to accept that after all of that, if you were still sickened and repulsed by him, he must let you go without making your life any harder.
I also think that if it was me in this situation, I would try in the meantime, to find if there were any sites for the partners of people who indulge in those sort of fantasy behaviours, and whether, and if so, how they came to terms with it, and whether their partner had completely stopped being on those sort of sites now, and if not, why were they still in a loving and sexual relationship with them? One last thing that I feel a need to add, is to ask you to remember that your (dear) husband is not only what you discovered about him yesterday, but also - most likely - all the other things that you do/did love about him, except that you believed him to be totally honest/open with you. Of course, and unfortunately, many of us have something in our past that we keep to ourselves, maybe something that we now feel embarrassed about, or regret, etc. Being a couple does not mean having to share everything with our loved one.
If your whole mind and heart have been rejecting everything I have said here, then you probably do already have your answer to what happens next... I wish you and your DD the very best of luck, and that the rest of your life is full of contentment and love xx