By definition, a lesbian is a woman who has sexual attraction to women (and not men). If you're not a woman (and are furthermore a man), then it'd be impossible for you to be a lesbian.
That said, maybe there's something you feel about being a "lesbian" that's not actually exclusive to lesbians. If you're big on rights for women, you are very capable of being a feminist heterosexual man, and you don't have to feel that support is diminished simply because you're a man. If there's some sort of expression or connection you feel comes from being a "lesbian," you can still have that same attitude or expression as a man (lesbians can have male friends like you!)
Alternatively, you may actually be a woman, and maybe there are certain things about you that you'd think would be for men but are legitimate for women to have as well. There are lots of possibilities for what men and women can be and what they can do, after all.
Alternatively, you could be third gender or genderfluid, in which case you're sort of splitting the difference, but basically you're not going to be neatly tied down as one of the two socially-recognized genders.
There might be more explanations, but basically there are lots of possibilities for who you are that do not require contradicting definitions.
Take your time.