In the scene in LW3 where Riggs and Lorna start comparing battle scars, he eventually grabs and kisses her. She quickly reciprocates. After they go to the floor, he pauses their kissing and mentions this could be a "serious ethical breach". She tells him to shut up and resumes kissing him. It looks almost like he was making sure their impending sex was consensual after his initial forward advance.
More likely it was a joke about her being in Internal Affairs.
Just how different are Riggs and Murtaugh? Their initials are reversals of each other (Martin Riggs and Roger Murtaugh).
Imagine being the poor police officer or ship worker who discovers Pieter after Riggs squished him.