I feel like "xianxia cougars" are basically all those stories where MC is a loser or maybe genius, just not average, and he randomly meets some powerful demon sect/evil faction saintess who fucks him dry over the course of 3 days/3 weeks/3 months/etc. and then 1-5 years later, she shows up and gives MC 1-5 kids and says they're theirs, or maybe makes MC care for her while she's preggers