A is people attracted to John; B is people attracted to Gareth. D, the circle crossing them, is people who are attracted to them both, so Jack/Ianto shippers, while C (attracted to John but not Gareth) are the Jack/Gwen shippers.

ETA: wait, I totally screwed up why D is Jack/Ianto, since different bits of it also include 'people who only fancy John', 'people who only fancy Gareth' and 'people who fancy neither'. This bodes well for the class I'm teaching on Thursday.