In the Buffyverse there are no male slayers. There are male watchers, but no slayers. I am curious as to why all the slayers are women? Is there an explanation that the show,comic or movie gave?
In-universe, there has been no full explanation of exactly why the Slayer Line only runs through women.
The spell used to create The Slayer and the Slayer Line also created the first group of Potentials - possible replacements for the Slayer. All of the Potentials were also all young girls. It is never explained if this was a conscious choice by the Shadow Men, something intrinsic to the spell itself, or a choice/preference of the Shadow Demon.
When Sineya later died, the powers of the Slayer were passed onto a random Potential.
Out-of-universe, Joss Whedon likes writing strong women.
*Note that all information is culled from the Buffy wiki. Some sections of the wiki are not properly cited, but the information is generally considered reliable.
For the same reason that the Shadow Men only made one Slayer at at time...if you are going to give great strength to one person, and you want to control that person, pick the gender that is strongly conditioned to obey. Doesn't mean that every Slayer will be a doormat, but it means they are less likely to go rogue, more likely to obey orders.
Buffy realizes this when she learns the origins of the Slayer line.