"Unlike with a woman, the passage you'd enter with a man is dry. You'll need oil to ease the way. Which... It's easy to find here. If you think you may need it, I can get some for you. Or tell you where to get it."

It's not like he and Wangji don't have a stash of lube. They could spare a little to save Sizhui or Jingyi the embarrassment of have to go buy it themselves.