star trek - What is this man doing with a Starfleet mini-skirt?

In the Star Trek: The Next Generation episode "Encounter at Farpoint", we see Deanna Troi wearing some kind of mini-skirt uniform.

I did not remember this before, as Troi wore more casual clothing later in the show, but I thought it was a great tribute to Uhura's uniform in the original series.

But later in this episode, we see a man wearing a mini-skirt uniform, just like Troi's:

While I did not see all of the TNG episodes, I do not remember anyone else wearing this kind of outfit. It's just that I wouldn't see Picard (or worse, Worf) wearing that!

I could imagine it's the next step in gender equality, but how did it come in the show and how it left?

Answer

According to Memory Alpha, it's a "skant":

Introduced alongside the standard duty uniform jumpsuit, a skirt or "skant"-style uniform was also available to Starfleet officers as early as 2364. Similar to its jumpsuit counterpart, the skant uniform was a short-sleeved one piece skirt, that could be worn with or without trousers, and included thigh high, or shorter, black boots.

It was worn by both genders –though men tended to wear the shorter boots –until it was phased out of service some time before 2365. (TNG: "Encounter at Farpoint", "Where No One Has Gone Before", "The Child", "All Good Things...").

It goes on to say:

According to the book The Art of Star Trek, "the skirt design for men 'skant' was a logical development, given the total equality of the sexes presumed to exist in the 24th century."

The uniform was used primarily by background actors, though "Encounter at Farpoint" featured both Deanna Troi and Natasha Yar in skant-type uniforms, the latter only briefly. While Troi got a new look entirely for subsequent episodes (according to the Star Trek: The Next Generation Companion the uniform was referred to as the "Space Cheerleader" look), Yar remained in her jumpsuit uniform for the rest of her stay aboard the Enterprise, never donning the skant after the pilot episode. Male crewmembers wearing the skant were only seen in few episodes, "Encounter at Farpoint", "Haven", "Conspiracy", "Where No One Has Gone Before" and "11001001", though the footage seen in the last episode was re-used from "Where No One Has Gone Before".

The skant was primarily seen in the first season, appearing only four times, worn with black trousers, in the second season episodes "The Child", "The Outrageous Okona", "The Schizoid Man", and "Samaritan Snare". After that its final appearance was in the flashbacks during "All Good Things...".