I NEED BOY ADVICE

Hi <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a> ladies!
I am in some serious need of advice... I have been with my boyfriend for almost two years now. Out relationship has it's downfalls, but for the most part has been pretty good! We have begun talking about marriage and having children someday. At first it was really exciting to talk about those things, but now when I think about the fact that if we get married, I will never kiss/sleep with/date another person again... That's kind of a scary thought when you're only 19 years old... Should I let my boyfriend know I may not be ready to settle down that hard core? Also, I have been feeling attracted to other guys. Do you think this is because I'm feeling this way, or am I actually wanting to be with other dudes? Please help!!!