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 Verhoeven’s… Verhoeven.”

In one of the commentary tracks on the American Blu-ray, the one with Paul Verhoeven together with Casper Van Dien, Dina Meyer and Neil Patrick Harris, they discuss this and Paul reveals that it were "the people who weren't there" who revealed that he had stripped on set: Denise Richards and Patrick Muldoon. He said they'd used it in an interview as yet another example of his depravity.