Should this be bothering me or am I being impature?

My husband has been divorced for 3 years and we have been married for two years. My husband has this thing the he does not mind being friends with ‘her’ as long as he never has to hear anything about her new husband, (she cheated on him and left him for another man) and that bothered me a little but I let it slide since she lives in a completely different state and the only things they do is e-mail, (there has been a total of 10 e-mails between in them in 3 years). But she just recently wrote him telling him she was coming in to town for a few days (who knows what for…) and that she would like to maybe meet but with him for lunch or something just to catch up, a casual lunch between friends. I know nothing will happen if he does go but just thinking about her makes me insane. I don’t mind that my husband is divorced, but it really bothers me that he was married, it makes me crazy jealous knowing that he was in love with someone to want to marry them, they where together 7 years, that’s a lot of love and a lot of memories…
Anyways, should I let it bother me? He told me about the letter the second it came in but my mother is in town for the weekend so we haven’t had a chance to discus it but I think he wants to do it. I don’t like this at all, personally I feel it is inappropriate for a married man and a married woman to have lunch with their ex spouses. Am I wrong to tell him he can’t go? I am trying to be reasonable and fair but at the same time I’m selfish, she had her chance, she should just leave him alone, he is mine now. Am I being immature? Should I just let him go? I am so lost… QQ
By the way there are no children invloved and I did not read his e-mail he told me about it and showed it to me. I know him enough to know he would never go behind my back about anything. My question is, when my mother leaves and we get a chance to actally talk, should I tell him I’m ok with it or tell him he can’t or what should I do?

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