According to the old Flash code, the list of valid races is: rodenian, half-rodenian, mouse-morph, mouse-boy, mouse-girl, rat-morph, rat-girl, rat-boy, rat-man, mouse-man; if your race doesn't match any of those exactly, it won't trigger. As such, it's possible it might be bugged due to a reworking of the race system. You also need to have genitals, not triggered the scene in the past 36 hours, and no more than 2 submission to the female SecOp; you also need to actually roll the female one, which is a 50% chance. After all that, the scene itself has a 50% chance to trigger, so effectively a 25% chance upon encountering a SecOp if you meet all the conditions.
It also has a Fight option, so you might've mistaken it for a regular encounter with them.