It is naive of both the man and the woman to assume that the other has always been a good Catholic. In fact the reason that one dates/courts the other is to determine if this other person is indeed as good a Catholic as he/she seems to be currently. And then there is the further question as to whether or not this person is a potential spouse.
One’s past is just one of many things that doesn’t need to be shared with casual aquaintances and may eventually need to be shared with closer ones. So I think it needs to be kept in perspective.
The subject came up in another thread as to how much you should tell your children about your past. Now children are not would-be spouses but I think one thing is the same. It’s better to tell your intended yourself than it is for them to be unpleasantly surprised later. Since family members and friends probably knew some of these people it is highly likely that they will not stay hidden forever.
I would say that in general the time to reveal the past (whether it is sexual in nature or some other problem) is when you decide that this new person in your life is someone you would want to marry and you think they have the same thoughts about you. I think that someone who wants such info too soon or someone who wants to share all too soon is showing evidence of desperation. And I think that is a red flag suggesting that this person has personal problems that need to be resolved before they are ready to commit to another.
But I also think that the time to reveal one’s past may need to be moved up if your past is really well known to common acquaintances. Likewise, if either of you is a public figure and there is a potential for scandal, it is only fair to let the other person know what they are getting in to.
As far a details are concerned, I don’t think it is necessary or proper to reveal all. But I think it is reasonable to give something of a timeline as to how many years you engaged in the sinful behavior and, perhaps even more importantly, how long since you’ve repented of your ways. (If it’s been a couple years, great! If it was last month then maybe you haven’t really changed.)
Names, addresses, and phone numbers of past partners should NOT be revealed. But you probably need to give a general count. (It was one or two; it was 10-15; it was 75+, …) If the occasional name or live person shows up then at least it will be within some previously known context.
And I just need to reiterate what others have said. If this new man cannot handle what he learns, he is not the man for you. It doesn’t mean he is bad or that you are bad. You are just not meant for each other.