A lot of people think a man can't be a lesbian, and perhaps only slightly fewer regard the topic as less than hot. I, however, regard it as very hot.
First, consider what it means for anyone to be a lesbian. To say that a person is a lesbian conveys information about the individual to the extent that s/he fulfills a role definable within the excercise of lesbian relations. It informs because it really means "s/he is an element in lesbian relations," where to be an element in a lesbian relationship is definable by the properties of that relationship. If there is no such definable role, then "s/he is a lesbian" can have no meaning. So the term "lesbian," when used meaningfully (i.e. to advance understanding of a phenomenon), ultimately derives its meaning from a type of relationship or pattern of relationships, rather than from a type of person. The case compares somewhat to referring to a gene by the trait it engenders, e.g., the "freckle gene." The gene does not, itself, actually possess the property of being freckled; calling it that merely conveys the information that it may, at some future date, under certain circumstances, give rise to something else with the property of being freckled. Reference to a "lesbian person" is the exact same sort of locution: by himself, the person does not possess the trait of being lesbian in a meaningful way. So just as one can understand perfectly what "freckled" means without reference to any gene or even acknowledgment that genes exist, to know what "lesbian" means one needs understand only a relationship--no reference to any type of person is required.
So: let us accept "lesbian," in all its common usages, as ultimately a reference to some relationship. The question remains, then: can a man be a lesbian, that is to say, can he have lesbian relationships? To frame the question formally: do any members of the unique set of properties which render a lesbian relationship distinct from the world's other things lack membership in the set of all properties which the relationships of men can possess? One example (which no one could take seriously) might be the condition that all agents in the relationship be biological females--clearly a property barred a priori to relationships of the biological male. It should be clear that the relationship must be incapable of possessing an essential property intrinsically, that is to say, by virtue of what it means to be a relationship of the biological male, rather than contingently, e.g., through such processes as cultural imprinting, however strongly such processes may in practice be linked with gender.
What is not required, however, is that we know all, or indeed, any of the particular properties a lesbian relationship requires, since a definition is not our goal. The aim is only to determine whether it is possible for men to have lesbian relationships, and one can validly argue in the positive by holding that, regardless of what the particular properties that define a lesbian relationship must be, such properties must all be of a particular type, and that no property of that type is excluded from relationships involving males. This is, in fact, exactly the approach I shall take. I propose that the properties defining the lesbian relationship belong exclusively to a more general class; that it is a class of mind and soul ("spirit"), so that the necessary properties of a lesbian relationship are properties of mind and soul purely; that the only determinant of which spiritual properties a relationship can possess is the imagination of the individuals in that relationship; that while gender may influence the nature of an individual's imagination, it in no way predestines it; and that therefore no one is barred from having a lesbian relationship just because he is male. (As a point of order I should note that it also follows that no one is barred from having a lesbian relationship just because his partner is male, and further that the principles derived above probably hold for every kind of human relationship, not just the lesbian kind. It is my hunch that there is a kind of universality of mind and soul such that if it is possible for one set of individuals to relate in a particular manner, then it is possible in principle for any like-numbered set to do so; that is to say, the kinds of relationships people end up in is not ruled by any law, but by the imaginations, wills, and other self-determining forces of the people involved. However, this is all somewhat speculative.)
It is worth noting at this juncture that the preceding argument can yield a proof even when only partially accepted. If one accepts that a relationship is limited solely by the imaginations of its creators, but holds that gender can predetermine imagination's limits (cultural anthropologists will note that this view is generally known, in the prevailing jargon of the day, as "sexist bullshit"), then one no longer has a valid proof that men can have lesbian relationships. However, I take it as self-evident that no one can know the limits of another's imagination; therefore, one still has a proof of sorts--a proof that no one can prove that a man cannot have lesbian relationships. This is an interesting result. Of course, if one accepts the elimination of gender as a predetermining factor, then the result is a bona fide proof that a man can have lesbian relationships.
All of which raises the distinct question of how one can tell when a particular relationship is a lesbian one. The research in this area is thin and not encouraging, and its discussion may attract existentialists. It appears that when "lesbian" is used simplistically to designate any female-only relationship that the problems are not great, since to ascertain the gender of one's partner is usually a simple matter; however, attempts by individuals to use the term more meaningfully, i.e., in a way intended to further understanding of their lives, invariably reduce to characterizations of spiritual value, and these have not yet revealed enough common elements to reliably distinguish the spiritual value of lesbian relationships from that of romantic relationships generally. So it then becomes an open question whether anyone can know that he or she is a lesbian, or in a lesbian relationship; the term "lesbian" may turn out to be completely meaningless (except perhaps to certain political groups, for whom its meaning appears to be non-scientific). This suggests the exciting possibility that we may all be lesbians and just not know it. I actually doubt this, however.
It should be apparent that the science of human relations is sketchy and contains much open ground. One fascinating area, worthy of further study, lies in the idea that there is a "deep structure" (in the sense defined by music theorist Heinrich Schenker and applied by linguists to the study of natural language) to all human relationships, of which particular types such as the lesbian are derivations. Indeed, the assumption that there is such a deep structure is what inspires scientific endeavor in the first place, so you can see why there is such interest in the study of this matter! Another fascinating topic concerns how one changes oneself in order to realize an envisioned and desired relationship. If anyone capable of love is capable of lesbian love (whatever that may be), what then is the procedure to realize that potential? ("Use your imagination," is valid but unhelpful.) The way new relationship types originate in the world could have implications in a multitude of fields: the discovery that they originate in response to the emergence of new needs in the individual would coincide with evidence that consciousness evolves historically. And was the first lesbian relationship an isolated case which then spread, or did it flower globally? In either case, the ways in which a type of relationship (or emotional need) proliferates in a community demand further study (it is false that at least one participant in a lesbian relationship must have had such a relationship before). And what kinds of interpersonal relationships are possible with high-order non-human animals? Is it possible to have lesbian relations with a dolphin? Research on these matters is scheduled to begin shortly, pending approval of federal grant money. It is my hope that these questions, unanswered and sketchy as they are, provide sufficient framework to indicate where current thinking in the realm of personal relationships is headed.