How should I reply to a way off base email from a jealous girlfriend?

I messaged a guy on facebook that I used to have a crush on like 7 years ago, just to catch up because I hadn't seen or talked to him since high school. It was completely innocent, although I did say I used to have a crush on him and how much of a dork I was. He was never interested, but was always really nice. I even congratulated him on his engagement with the girl and mentioned my happy long term relationship. Today I got a message from the fiance saying that she read the messages and that she doesn't want me talking to him and sending messages about the huge crush I had and how hot he is (exaggeration) because they have jealousy issues. I had no inappropriate intentions and am a nice and quiet person, but I don't want to just reply 'ok' because I don't want to be a doormat. What should say that is tactful and takes the high road, but also doesn't come across like I think I was in the wrong? Please help!