Male goths wear two types of skirts. It's not considered
cross-dressing.
The first type is always plain black and floor length. It's usually worn with
black boots, black tights and a man's shirt.
How did this start? Based on the limited types of skirts seen on the
dance floor, they might be reminiscent of priest's vestments (spooky spooky). It's entirely up for speculation.
The second type is the Scottish kilt and it is usually as close to
traditional as possible. This can be rather expensive unless you're
willing to wrap your own.
There is a lot of gender-bending in the goth scene, and the truth is
that anything goes. For example, there are heterosexual guys who dress
like girls, pass for girls, and who have no trouble getting dates with
girls. However, the two types of skirts described above are
exceptional in that they are not considered femmy.