Um...
...provided things are the same for ladies as they are for guys the only person who really knows is you.
I'm wondering why you jumped straight to lesbian and weren't pondering if you were bi or not?
I'm bi and at no point did I consider the possibility that I was gay.
Now there is one big difference I suppose and that is the opposite sex for you is fat, hairy, sweaty and smelly whereas for me it is soft, gentle, beautiful and tender. How can women not be attracted at least a little bit to other women? The wonder is that any guy can be gay or bi!
Getting to the point here you are a in a relationship with a guy yet you are attracted to women and jump to the conclusion not that you like both but that you only like women? What does this mean for your relationship with guys? Are you actually attracted to and sexually aroused by men? If so Charlotte it is pretty safe to exclude lesbian from the equation.
The next bit would be what does it matter and/or what are you going to do about it? You are aroused by the thought of women too (or only by them). You can just carry on in your relationship and use this knowledge to help you along when you are rocking the man in the boat or lying back and thinking of England because DP isn't quite on the right bandwidth; you can take it further and talk to bi/gay ladies online; you can meet up with bi/gay ladies and see what happens; you can do this openly or on the quiet; you can dump him and see what happens when you try the said meetings etc; or the two of you get into swinging as I am informed that bi-play between the female partners is quite common.
The ball is in your court. I would say though that you know. You know why you said gay not bi (whether it was a slip or deliberate), you know whether or not you are gay or bi, you know if you must experiment or not. You have to be yourself otherwise you may explode. That doesn't mean you MUST go out and cheat with some lassie or dump the BF and shack up in some commune but it does mean you just admit how you feel to yourself and go from there. You don't have to act on it (unless you truly realise you are a homosexual trapped in a heterosexual lifestyle and even then it is up to you).