Why is forgiveness even needed? What each person did in his or her past is that their past. God forgave them and remembers the sin no more.
God's forgiveness doesn't mean there is no further need for human forgiveness. David used hyperbole when he said his sin with Bathsheba was a sin against only God. The sin against God is indeed the main thing; but sin can be against other people as well.
I know mpwhen my husband and I mentioned our pasts... We didn't say forgive me or whatever, it was just a this is what I did/involved in. My husband wasn't a virgin, I wasn't upset about that at all. I wasn't upset that he didn't save himself for me. It was what it was, the past.
Possibly you expected it would be likely that you would marry someone with sexual experience, and so you were OK with that. You also didn't see it as a sin against you. Me, I wanted to marry a virgin (I didn't) or a widower without the baggage of sexual sin (which is what I got). If he had told me that two or three years before we met he had been involved in sexual sin, it would have been huge. I would have had some real forgiving to do. Depending on how serious the sin was, I might have walked away (it would have said something about his level of spiritual maturity if he had been involved in sexual sin in his forties when he was "definitely old enough to know better"). If the sin had been before his first wife, years ago, it still would have taken some dealing with, but I think it might have felt more like a sin against his first wife than against me specifically. But sin between her and me would have felt like a sin against me, and a serious one.
But everyone deals with things differently and has different levels of what sorts of baggage they can handle. Some people can't handle spouses who are poor, or have serious health issues, or who want lots of children, etc. What is a big deal to you might be trivial to someone else. According to Scripture, sexual sin is a really big deal, a very serious sin . . . and it's fair enough for a future spouse to have some real struggle with it. But what matters now is how he deals with it now that he knows and has had time to process it.