Those wicked female flatmates. So much visual teasing could drive a chastity guy to distraction. You should revisit them after a few weeks... or months.
Flatmates interest me greatly, because of my own experiences, but also because of the 'thrown together' male/female dynamic. Sometimes it will be flatmates who lock, other times it will be flatmates who simply get turned on by the face that their flatmate is locked by someone else ... but either way, the fact that they don't come into their contact with the victim as 'a couple' provides an interesting angle.