I don't get too many creepy whispers or OOC actions on this server. Most of the time, when someone hits on me, it's IC and pretty harmless. However, there have been a few times when the other player (IMO) crossed the line and started to make me genuinely uncomfortable. Where do you think that line is drawn in terms of playing a flirtatious or even perverted character? Usually I just react ICly by making threats, telling them off, or ignoring them (depending on the character), but there's a point when OOC communication is probably necessary just to make sure the other player(s) is ok with your behavior. This goes for other types of RP, like having a particularly hateful or confrontational character.
I don't mind flirting, but when a character starts making excessively sexual comments or starts following me around, that gets a bit creepy.