Just because you have fantasies about the same gender does not make you lesbian.
Even going through and doing something with someone of the same gender once or even twice does not make you lesbian!
Being lesbian means that you feel sexually attracted
only to girls. You only want to date girls, you only want to have sex with girls, and the thought of sex with a male just doesn't turn you on. A lesbian may even consider kissing a guy to be icky, gross, and a turn-off.
You sound like you are curious about sex with the same gender. This is often called bi-curious. There is nothing wrong with feeling this way. If you feel that you wouldn't mind doing something with a girl, then go for it: but only do it for
yourself, and never because your boyfriend wants you to do it!