Why are you even considering getting married at that young age man?! These are your best years, get out there and use them before you're too fat old and ugly to get laid
If she wants to go out with other people maybe you should consider the same thing. Dip your scoop into the honeypot of a few girls and try some shit out
Seriously if you let her do this to you you're going to be her bitch for life. She'll do it all the time and she will probably not tell you about it in the future.. she'll just consider it to be "ok" to go and fuck around with other guys. Do you want that? Also, how do you know that this guy she wants to "try out" doesn't have some STD which your girl will then bring back to you at some point? Fuck that.
Really that whole situation is just messed up. You wont get out of this relationship what you want to get out of your ideal relationship. You'll just end up her little whipped bitch.
Kinda sorry i come off as blunt as i do... don't mind the swears.. i'm from NJ this is just how i communicate
Edit: btw how do you know she isn't already messing around with this guy? or other guys? It's not worth the BS man, just run while you can.