In my own expanded head canon, a "male slayer" is impossible. I believe (in head canon still of course) the wizards shown in Season 7 who initiated the Slayer line and pretty much founded the Watchers tried that first, as their "natural" preference.
In terms of real world materialistic sociology, I actually believe that evolving as gatherer-hunters, our species completely lacked gender dominance. Not gender polarization; all the anthropology I ever learned affirms that the roles of pragmatic day to day economics of gatherer-hunting were quite rigidly divided. Grown up men were not supposed to gather--maybe some casual plucking of vegetables of various kinds, nuts and so forth, for self-sustenance, but not to bring any surplus for others home; similarly women did not systematically hunt; one account of some young girl in a southern African desert society stuck with me, of a girl who boasts of killing a "dik-dik" (a very small kind of deer) dispels any notion that GH society had fierce and violent enforcement of these divisions; it was perceived and experienced as a pragmatic thing. In general there is little sign of humans terrorizing each other prior to the development of cultivation and new stressful dependencies on constraints that did not apply to our ancestors. (This was possible for them because they lived in small numbers). So I believe that in the materialistic real world, there is no sharp line, just a spectrum and a highly overlapping one at that, and that societies benefit from both men and women being regarded as equally well able to do anything.
But the Buffy'Verse is a different place. In it magic and mysticism are fundamental, material if you will, realities. Per what Buffy and Angel canon eventually revealed and guided by Giles's Watcher trained exposition, it seems plain that there is a metacosmology, which Giles in an early Season One episode, perhaps even Welcome to the Hellmouth part one, expresses in terms of Earth alone, but I take on a grander scale. The meta-Cosmos was originally a multiverse of "hell dimensions" inhabited by "demons" of unknown origin, perhaps eternal, who by nature are creatures of Will. Our species as I interpret it is a kind of subspecies of "demon" that as it were migrated from the central zones of contention of raw power, being defeated as weaker, and sought refuge in a "crystalline" sort of hedge of increasing lawfulness. We increasingly submit to grand and abstract laws we conceive of as inherently beyond our power to alter, regulating all, and that true demons have trouble navigating in. They can use raw power to batter their way in as it were, but it is increasingly difficult and unrewarding for them; they don't "belong" here and successful demons can't be arsed to try, the pickings are slim for them. Metaphysically, we as a species have been migrating away from the power of raw will and so have been, in terms of parallel universes, into timelines where an increasingly materialistic reality is developed at the same time as we evolve elaborate societies based on increasingly impersonal social laws.
This is why magic is so dangerous and increasingly rare. Fundamentally, using magic surrenders some of our advantage and has us playing more on demon turf. The Slayer has the mission of standing at the rear of our species's advance away from the chaos of raw will into the realm of ever more rigid law, posting a guard against demonic forces that would seek to draw us back. We have behind us a comet trail as it were of various hybridized demons in zillions of alternate realities that are either snapping at our heels or also co-adapting. The Slayer is the keeper of the border.
Note that in Buffy/Angel canon, witches who are basically benign or anyway decent are pretty common, though as Tara told it, they need a lot of collective communal guidance and grounding and have strong codes of morals and ethics to guide them. But you almost never meet a male wizard who is decent; Giles is a creature of great self discipline and restraint. Projects like The Initiative, trying to mix a bit of magical fudging with dominant power structures of human society, are doomed to go wonky, in great part because again people are playing on demon turf and the demons have the advantage. But also, dominant human power structures tend to be patriarchal.
In the Buffy'Verse, a lot of propositions that deem questionable and even dangerous in our actual world are simply factual. There is such a thing as "male energy" versus female energy. The Watcher's Council is predominantly male-patriarchal in governance and mindset, and is therefore somewhat corrupt, along similar lines to the Initiative.
I think it should be plain, or anyway a reasonable supposition, that the all-male and quite ruthless council of primordial wizards who raised up the Slayer line by incorporating a "male energy" type demon into a Chosen girl, essentially violating her as Buffy perceived it, would have attempted to raise up a male Champion for the role first. It is my supposition that they tried this many times, trying their best to fix problems, but always found they had created a Frankenstein's Monster in doing so. Either the male "Slayers" had to be forcibly separated from their demon imbued powers at great trauma and possibly generally fatal in itself, or hunted down and destroyed, one after another, like so many Skynets. Only as a last resort did they fall back on the desperate expedient of trying it with girls, and much to their annoyance, this worked--eventually anyway. The wizards resent this and wish it were otherwise, but they remain ethically grounded enough to accept it as unfortunate but true. It was as much in jealous spite as anything else they decided on the One Slayer, She Alone rule--there may have been other considerations, but I think the fact Buffy gets away with breaking it, at least apparently, suggests there was no real reason there should not have been lots of Slayers at a time.
So, I think any attempt on any man's part to become a Slayer or for anyone, man, woman, a pangender team, to make any man a Slayer, is doomed to lead to catastrophe and regrets in the same way the Initiative was so doomed. A sane and moral man would seek to reject the graft despite the abject misery being desolated of their stolen demon power would cause, and more typically we'd just dive right into it and be consumed much as Dark Willow was, only with essentially no road back we could be expected to survive, and if by some miracle one of us survived we'd be quite wretched and wrecked. More typically the Slayer or an army of them would have to destroy us or we'd do great damage to humanity.
Obviously other people have other notions, but this is the canon as I have come to understand it. No male Slayers!