It’s not the crime, it’s the cover-up. This is true for all guys (including presidents). Don’t lie to your girlfriend about where you were or who you were with. I certainly wouldn’t lead with that information though. And I wouldn’t offer it up openly, but definitely don’t lie. Depending on how good of terms you are on with your ex, this all boils down to the fact that you were shopping for an engagement ring for your now fiancee, so how much of a bad actor could you really be in theory after all? Apologize, because this isn’t exactly your greatest idea, but after that move on. The most important thing is to remember that you’re getting married now. And maybe lose that ex’s number.