Most likely that (as posted on Wikipedia):
The cast agreed to do the co-ed shower scene only if Verhoeven agreed
to direct the scene naked, which he did.
The Wikipedia article refers to the DVD commentary as the source of this fact.
Verhoeven confirms the story in this interview with Digital Spy:
One day's filming that didn't make it into the Making Of is an
apocryphal incident during the marines' mixed-gender shower scene that
Verhoeven happily confirms he stripped off for too to soothe his
nervous young cast.
"Yeah, sure I did that. And it helped!" he says proudly. "My DoP Jost
Vacano and I, we undressed and of course then everyone started to
laugh. Then we shot the scene with no problem. Jost grew up in these,
let's say nudity camps [sic], so he had no problem with it. For me, it
was a bit more difficult but I said to Jost, 'Hey, we need to give an
example...' He said 'Of course,' and boom!"
He told Empire magazine a similar story:
Unfortunately a few members of the cast detected not just a subtle
political and psychological critique of the authoritarian mind but
somewhat typical sensationalism from the director who had, after all,
delivered Sharon Stone's infamous leg-cross in Basic Instinct only
five years before. "One cast member said they would only get naked if
we did," says Verhoeven, "Well, my cinematographer was born in a
nudist colony and I have no problem with taking my clothes off, so we
did. It is strange, but of course Americans get more upset about
nudity than ultra-violence. I am constantly amazed about that. I mean,
I haven't seen any sex scenes in American film that are anything other
than completely boring. A bare breast is more difficult to get through
the censors than a body riddled with bullets."
There is also this report from the 2016 Toronto ComiCon:
At this weekend’s ComiCon in Toronto, Casper Van Dien and Dina Meyer (aka Rico and
Dizzy) confirmed the long-standing legend that the cast refused to
strip down unless their director did the same (albeit Verhoeven got to
stay behind the camera while his clothes were off). A lot of people
would prefer to face an entire hive of Arachnids before baring all on
the big screen but, as the 20-year anniversary of the film approaches,
Meyer and Van Dien remain unfazed by the experience of being nude
among their co-stars. Seeing their director naked is another story.
“It was probably about 100 people in the room,” recalled Meyer, “and
Paul’s like ‘Before we start shooting we want you guys to get
comfortable with the nudity.’ So he basically had us do a bit of a
fashion show without the fashion. We were a little reluctant to
disrobe, and Paul was like, ‘I don’t understand you Americans! What’s
the big deal? Just do it, it’s fine, no problem!’”
“And Dina,” interjected Van Dien, “says ‘Paul, if it’s no big deal why
don’t you do it?’ And so he dropped his drawers and all of us were
like ‘Augh, god! Dina! Why!?’ So thanks to Dina, we’ve all seen Paul