I have a crush on this girl (I have a boyfriend)

So theres this girl that I really like. Shes super cute and fun to be around but im in a serious relationship with a guy. Ive asked my bf if he would be up for a threesome but he wants me all to himself. Im in college and I really want to have fun but I know hes the one I want to be with for the rest of my life. What should I do ?