I would say that's something you've got to work out and be comfortable with. I started to question my sexuality at 13 I think and it took me a few years to figure out what I thought I was and what my priorities were, so don't worry about being confused at your age, it is a process that we go through and however long that takes is obviously individual.

How do you like guys? Is it actually sexual or do you feel something else for them, like romantic thoughts (by that, I mean a desire to be in a relationship bond with them) or being attracted to their aesthetics (how they look) or something else? How do you like girls? I don't know from what you said whether you aren't attracted to girls outside of your intimate time or you find the prospect a little repulsive (from a personal viewpoint) outside of that time.

The human brain is a complicated thing. I'm sure more people have odd fantasies during sexy time than they'll admit. There are people who see themselves as the opposite sex during fantasies, for example, when they wouldn't normally consider themselves transgender. If it's restricted to that time and you don't otherwise feel romantically or sexually interested in girls, I'd sort of make a mental note of it but identify as hetero if that's what you are comfortable identifying as. If you later decide you actually are gay or bisexual then you can revise your identification as you see fit. At least with hetero you are naturally assumed to be so until you declare yourself otherwise, so you could sort of stealth until you've worked it out.

I know that working yourself out is a confusing and often distressing process, but no one can tell you what you are, it's a matter of looking deep inside yourself, being honest and finding your own evidence. And then of course learning to come to peace with what you find there.

Good luck