Although I would never advocate for two people who are unequally yoked in matters of faith to get married, and I think the OP is clearly expressing doubts about moving forward, I think some of you are unfairly vilifying the OP’s SO.
The world is filled with people of different faiths (as well as those who are agnostics and athiests.) Many, if not most, of those who sincerely practice a faith are not also teaching their children that all faiths are equally true. Many people are sending their children to receive private, faith-based education or instruction, either instead of or in additon to public schooling. The instruction they receive there is not likely to be telling them that all faiths are equally true.
The OP has made clear that her Fiance does not intend to shelter his potential future children from the fact that other religions exist. Of course they will learn about that. What she has indicated he is saying is that he wants to raise them in his faith, which happens to be mutually exclusive with other faiths.
If one subscribes to Biblically based Christian doctrine, the chief foundational truth of that faith, from Genesis to Revelation, is that all of humankind became separated from God through sin. That when one person sinned, all became guilty of sin. And that God sent his only begotten Son to die on a cross and to shed His blood as an atoning sacrifice in our place. That salvation and redemption and reconciliation to God are available only through belief in and confession of that truth. In Christianity, if there is any other way for humanity to be saved, God would not have had to come to earth and die in our place. That baby, that man, that cross, and that blood is, according to Biblical Christianity, THE ONLY way for humans to be forgiven and reconciled to God.
The OP’s Fiance is not being narrow minded, uncompassionate, awful, bigoted, terrible or unfair. He, like many other people of many other faiths, is simply trying to adhere to the tenets of his faith. One of those tenets is that he must bring up his children according to his faith. People of different faiths from all over the world do this every day. They still manage to be compassionate and caring toward others who believe differently than they do as they interact with them in a pluralistic society. But that does not mean that they must or should allow that pluralism to infiltrate their faith. Sadly, so many people today are allowing this, and their alleged adherence to their faith is far from the truth of that faith.