Yes, it would be wrong. You know it's wrong, but you asked the question anyway. It's wrong, specifically, because your motivations are suspicious. Why would you want to remind this dude of your carnal past together? Because you want to rekindle the romance. Or, at the very least, you're bored and horny and want a little reminder of what was, and honestly, won't be again?

Don't tell him how great he was in bed. Nothing good can come from such flattery. Chances are he remembers exactly how great the two of you were in bed. But he's in a relationship now, and his focus should be on the present and the future. Not the past. Let me ask you something: did your ego, in a moment of selfish, lustful cruelty, write this question?

I am sympathetic to unrequited love. It's why I have a deep, abiding love of old school country music. Because it's about the one that got away. And we all have one that got away. Someone who we'll always keep locked in the basement of our hearts, who we'll always wonder "What if." There is an inherent bittersweetness built into love, because we can only control who we love. We have no control over who loves us back, or how they love us back. Or when they do, if ever. Dealing with losing love is just one of the fantastic pangs of pain that we all feel. It makes us human. Be thankful you had a connection with someone. Many people go their whole lives without ever knowing what it feels like to feel someone in their bones. Never regret loving and losing.

I'm also sympathetic to being bored and horny. But surely, you can find a plaything who isn't involved with someone else.