I think it’s important to keep in mind that as humans we are imperfect and subject to temptation and mistakes. A lot of times we make mistakes in our past, and present, either because we fell in weakness to temptation, or at the time we weren’t in full communion with the teachings of the Church and didn’t understand the error in our ways. I am not married so I cannot speak to that question. But I know, due to my own failings, that temptations of the flesh can be incredibly difficult and if you are not vigilant about what circumstances you are putting yourself into, it is easy to fall to varying degrees. I think the important thing to look for is that there is sorrow for the faults we have committed and an intention to move forward in a better and stronger way. Personally, I don’t believe I can hold the shortcomings of a partner against them because I know that in my own ways I have failed in the past and would not want them to hold them against me.
Mary Magdalene was known as a promiscuous woman, and yet Christ loved her and welcomed her to Him with open arms because she came to him humbly in recognition of her failings and with a desire to be a better, holier person. I think this example speaks volumes. Whether the shortcomings/sins/mistakes of our spouses/significant others were sexual or not, we should not dwell on what they may or may not have done in the past, but rather on how they are striving to live their lives now. St. Augustine, one of the greatest saints, in my opinion, also had a sinful and sexual past, and yet he is now a great saint. It’s hard to not think of our significant others sharing that bond with others, but I believe that if we focus on the love they are offering us now, that the people they were with in the past and the mistakes they made in the past, should be left there. And I can speak from experience in that being constantly reminded of your sins of the past and how they hurt the person you love doesn’t help you move forward spiritually either. You end up miserable or hating yourself and getting stuck in your past sins because of the effect they are having on your present, rather than being able to progress. And, assuming you are a decent human being, you hate the thought that something that is now out of your control, and yet at the same time your fault, is hurting someone you love and impeding their desire/ability to be with you. You want to be able to change the past for their sake, but you cannot.