People are allowed to have personal memories, because we are all, always, ‘in the middle of things’ as they say.
Unless the actions of his past will affect Sally, then he isn’t sinning. He might not consider it an issue. Perhaps it was a foolish teenage thing, or he was abused. But if he was loose and contracted some disease like herpes or HIV, then he is obligated to tell Sally because once married, they become one in the flesh - and she could contract it. She has every right to know about this.
He can’t forget something like that, and if she wakes up one morning and he says to her “oh, by the way, my duck, go get your HIV’s tested. I had a run in when I was sixteen” then he’s not really a proper husband.
However a lot can be said if Tom doesn’t WANT her to know. She really doesn’t NEED to know, but it would be virtuous that, within the union of marriage, one emptied his heart to the other - for God’s grace prevails within the sacrament of marriage.