I think it’s the high waist cut for the yellow area. Plus the thong in the back. In fact, the yellow area looks a bit more like Borat’s swimsuit with a belt. The high waist was starting to be a trend irl back in the early 90s but was still seen as risque back then because of how much hip was shown. Thongs were rarely seen in public.

Traditional superhero outfits had more of a “briefs” look which were just seen as more normalized. No high waist cut, and definitely no thong. Yes there’s blue spandex underneath so no actual skin is shown. That might be the only way they could have gotten away with it from a publishing standard. But that particular cut was just more suggestive than previous ones based on societal standards of the day.