Wow, clingy much?
If I was your fiance and I saw this thread it would definately give me second thoughts.
If you're getting obsessive over him doing something without you now how much worse are you going to be when you're married and you feel he owes you 100% of his free time? Which, btw, he doesn't. If you can't do things apart without worrying you will have a bad marriage.
Maybe he wanted a night out with his mate, as a wingman, and you would have been a cockblock (or killjoy in that specific situation- would you want him there in every single situation you enjoy with your friends? Not a bad thing). Or maybe he just wanted a night out without his fiance, what's wrong with that?
It doesn't mean he doesn't want to have nights out with you- just that he doesn't ONLY want nights out with you. Going out for drinks is different depending on who your with and maybe he needed a different night?
And if you're now assuming a different night means he's keeping things from you, or that he shouldn't feel like he can't do some things if you're there; or even that if he's doing things that he can't do with you he shouldn't want to do those things, then
You are a ****ing terrible girlfriend and fiance, with very little understanding of your partner.
Get over yourself, stop feeling rejected and start getting your own life so you don't end up jealous of your fiance.
Marriage and relationships is about sharing your lives not merging them.