You don't have genitals and Piccolo was always canonically uncomfortable with human sex and thought it was strange/weird.
He'd have been a hardcore asexual in human terms, were he a human.
Beyond that is the fact that Vegeta was generally stronger then Piccolo for most of the series and so the window of time Piccolo had to actually have non-consensual sex with Vegeta would have been very small and given the events transpiring during those periods and the fact that they may not have even been in eachothers company during those periods the chances of the rape occurring would be very slim.
