Pansy and Draco had struck up a deal the previous year after both losing their virginity to the other. Pansy bluntly informed him afterwards that she had been pretty sure she fancied girls, but that sex with Draco confirmed her suspicions. It was easier for both to pretend to be in a relationship and appease their parents, and when the time came, Theo and Draco felt comfortable confiding their relationship since she was essentially in the same boat. Blaise caught Theo and Draco snogging in a broom closet, so that was that.  