I think my girlfriend is perfect for one of our best friends, help!

Okay, so I'm using my friends account here. I'm in my twenties and male. My girlfriend (D) and I were a love at first sight kind of thing. We met at a concert and started talking and I ended up bringing her home. We became best friends immediately and she came touring with me and my band. A month after meeting we started going out, we've only been going out for a month and a half and today her and one of my best friends (B), who she quickly became great friends with too, were messing around and I had the sudden thought that they were perfect for each other. I also felt a tiny bit jealous, because I love her but today made me think that maybe not like that. To complicate things further, a week after D and I started going out my best friend G told me that he loves me. The thing is, I love him too. He's my best friend and practically family but I don't know if it's family as in brother or family as in spouse. The thing is, I've always thought I'm straight and if I like G then that means I'm not. I'm kind of panicking here and I guess what I'm asking is, is it okay to think your girl/boy/date friend would be good with someone else? Or does it mean that I don't love D like I thought I did?