Hi Hakuna,
Don't pressure yourself so much about who should be first. It will be your "first" time with a girl, and your "first" time with a guy. And no matter which happens first, it's not going to permanently effect your orientation. It may sway it one way or the other, but whatever you are attracted to will always remain.
Let me ask you this...IF you find out that you are mostly or completely straight, what is going to matter if a guy was the first, or second or third experience that you had? You can visaverse that with the same question.
I'll also let you know, that I don't particularly remember my first encounter of either gender.
Relax, take the pressure off, don't worry about performance, just enjoy it when it happens.