First off, it's a totally normal fantasy to have and I'm sure everyone at some point in time has questioned their sexuality. With your experience though, it may have been wrong for you because you went about it the wrong way. You said that it was your first time with a guy, and you were in an uncomfortable position, and you had just met him. All 3 of those could be factors saying that you could still be bi (I'm sure you're not 100% gay since you enjoy sex with women) and that just wasn't the right experience.
You should really try something like this with someone you know more than a stranger or trust, and in a comfortable setting. I'm not saying rose petals and a hotel bed, but not necessarily in the back of a car.
Also, think about the experience more closely. What about it worried you and what part about it did you like? Did you, for example, like kissing the guy, or did that gross you out? Picking out what was wrong and right to you could help you decide if you don't like guys or you just don't like that one.
Another thing you brought up is that this a fantasy and you've masturbated over it. Like siempre nada said, there are people that are turned on by things like rape fantasies but don't actually want to be raped. However, couples have roleplayed rape scenes and things like that that are consensual but let the person act out their fantasy, which is totally okay, so don't rule it out.
I think when you're ready, you should try it with a guy again, but be more wary. Find someone that you are attracted to and turned on by, and then the sex may be more comfortable. If you have follow up questions I'd love to help ^^