Hmm, do you mean Greco-Roman statues? Because many Greek statues were naked, too. I suppose I'll answer this question from the ancient Greek's point-of-view because it was with the Greeks when heroic nudity became popular, and I suspect that the Romans copied the Greeks. Correct if I'm wrong here.

Nudity was seen as a "costume" for Greek art while nakedness was a vulgar depiction of humans. Nakedness, for many cultures, like the pre-Classical Greeks and Near East cultures, were symbols for vulnerability, weakness, helplessness, and provocative passions which they thought were connected to evil magic. They were essentially finding ways to explain eroticism and arousal when seeing a naked body. However, ancient Greeks gradually developed an admiration for nudity. It was a symbol of bravery, athleticism and the grandeur of being part of a hoplite-centric society. Ancient Greeks also admired youthful boys (kouros) that were still in their adolescent age when they didn't have any pubic hair. Perhaps those are the ones you're referring to, because there were plenty of statues which did have pubic hair. The most famous early Greek sculpture depicting nudity and adolescent beauty is the Kritios Boy. It's also the first statue to use a contrapposto pose.

When the Romans captured the Greek city states, I'm sure they would've enslaved some sculptors and emulated Greek art at the time.