In order to answer the question "Can a man be a Lesbian?" One first has to answer the question "What is a Lesbian?" There are many ways to answers this question depending on what framework one uses. However if we examine these differing frameworks (using a meta framework) we can separate these frameworks into two primary groups.
The first group is that of positively constructed identity; these are frameworks that set out to say what being a lesbian is. Another way to describe this group would be as essentialist, there are essential aspects of a person that make them a lesbian. The other group though is negatively constructed, they are focused on what a lesbian is not. This is related to the idea of queer negativity which is an aspect of queer nihlism/queer pessimism.
So if we aproach the question with an essentialist lens, of what a lesbian is, a common place to begin is to start with the essential aspect that a lesbian is a woman, but that then requires us to answer the question "What is a woman?" And here again we are faced with the same split between positive or negative construction and the multitude of frameworks that fit within each.
One of the common ways to answer this secondary question today is through the framework of biological essentialism. Which is a framework that constructs certain biological facts that make someone a woman. However just in this definition we can see that this is not a framework that identifies the biological aspects of women, but instead attributes the title women to those with certain biological aspects. To restate: a framework does not describe what an identity is, instead it defines what an identity is; it's prescriptive not descriptive.
Because of this, without a framework there is no answer to "What is a Lesbian?" There are no essential natural truths as to what a lesbian is, but instead only frameworks that aim to answer this question. You may believe your framework is "the essential natural truth", thus is especially common among biological essentialists but ultimately there isn't one, only frameworks.
Finally to give my own answer to the question "What is a lesbian?" I would answer; anything. Anyone may be a lesbian, perhaps even only at certain times. Even things can be lesbians. Pretty much every dildo I've met has been a lesbian. I've also met men who are lesbians. Personally I consider myself somewhere between a male lesbian and a female fag; depending on the day of the week.
I'll let you figure out what kind of framework I may be using.