WANTS A GIRLFRIEND HAS A BOYFRIEND

In a loose sense, the title sums it up. I have an awesome, incredible, amazing boyfriend that I love to death and would really enjoy marrying, but I'm 19 years old, bisexual, and seriously think I might be polygamous. I have always, always, always loved deeply and wildly and truly think I'd like to experience more with my own gender. Girls make me absolutely melt and, even if it would just be sexually (in respect to our emotional relationship), I have NO idea how to bring any of this up to him.