Relationship with a girl

Honestly the thought of having a relationship with a girl is really appealing to me, and it's something I have wanted to have since I was about 12. However the most I have done is kiss a girl. Once when I was 14 and once when I was 16 (about 4 months ago). I'm with a guy at the moment and I love him to pieces. He is my whole world and I do intend to spend the rest of my life with him. However, this thought of always wanting to be with a girl and have sex with a girl is really playing on my mind. I don't intend to cheat on him - because I love him. But I kinda wanted to live a bit more before committing. I suggest to him a threesome and he went crazy at me saying how it was 'cheating', so I don't really now what else to do, or how to get over the thoughts I've been having. Any suggestions please???