I don't know. It's just a dance, not sex. A dance in front of hundreds (maybe more) people. If they were being set up on a date, yes, I'd find it offensive and disrespectful. I recently started dating a man who has an ex-wife and they are pretty friendly. I am not the least bit offended by it. I have been around them, and don't feel threatened by it at all. I understand people can be great friends without having feelings of lust or love anymore, or realize there is no possibility of a relationship due to addictions or mental illness because I have been there. I am friends with ex-boyfriends and have no romantic or sexual feelings whatsoever. They're just people I was friends with before dating, or people I find easy to talk to, so why should the ending of our relationship be the end of our friendship or ability to get along? Learn to trust, if there's no reason for you to doubt him, then don't accuse him of wanting to get back with her.
Besides, the daughter isn't excluding you from the wedding, as if you're a nobody to her and her father. They are wanting your presence. It is just one dance and the rest of the dances are for you and your boyfriend to share together. Have you not danced with a friend or family member that was married, ever? Sometimes people ask you to pick random people off the dance floor for a dance (or someone picks you up), doesn't mean it's anything more than that, there doesn't need to be inappropriate closeness or touching. A conga line isn't something intimate or a reason for a spouse to blow up, for example. While it might have been offensive to admit they are hoping mom and dad get back together, I think all kids of divorce secretly wish this, however unrealistic it may be.
Trust your boyfriend, after all, it takes two to tango and he isn't interested in tangoing with her, but with you. They don't even live in the same state, so what is there to worry about? He is right, he is going home to you and making love to you. Isn't that good enough? If he really wanted her back, he'd ask for that ring back and dump you. He hasn't, so why fret over a dance and imagine all these crazy scenarios where they end up together? It's been over 20 years that they have been apart. I highly doubt he still has feelings of love for her or he would have dumped you the minute he knew she was single, and would be arranging for ways for her to move to Florida.