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Yes you are right it is his personal business and was before you so you really shouldn't tell anyone. It's hard enough for him to sort out his feelings and sexuality without having the added pressure of having to explain to your friends and family something that is so private. As far as your fears, I think it's fair so say that you are afraid that he may figure out that he wants to be with men again and may leave you for them or perhaps it could be that you do not want to get married and have children and then later on him come home and tell you he is leaving you for a man. You can usually judge a man by his track record in relationships and like he has told you he has always been in monogamist relationships so you don't have to worry about that but your worries are something you are definitely going to have to talk to him about, tell him your fears and be completely honest he will respect the fact that you are being honest and willing to work through this.
Many people are bi-curious and want to know what it's like to be in a gay relationship but the fact that he was with someone for a year and a half may make it seem a little more gay than bi-curious. He has to be the one to put your mind at ease because this is someone you want to spend the rest of your life with, maybe ask him to go to counseling before he decides to take such a huge step with you this way the counselor could help him to figure out and come to grips with his sexuality you would rather find out now than later when you are married with children. Make sure you let him know that you are going to be supportive of him no matter what because struggling with your sexuality is not a fun thing to go through especially alone and when you have a great support system it's easier for him to work things through.