My SO says it's okay πŸ€”

So my SO knows I like girls and he says I can have a girlfriend. But we're engaged! I'm fine w/ suppressing my urges for things I can't have, done it my whole life. But it is really making me wonder why he would say it's okay and he wants to get married and soon at that. His reason is that he doesn't want me to be unhappy and that he knows it's something I want no matter how much I deny but i just feel like if I do that he will use that as an excuse to do something crazy. Ultimately I told him no and that I feel it wouldn't work. I guess I just want to know, why would he want me to do something that I know he's not fully okay w/?