鸳鸯戏水

鴛鴦戲水
yuānyāngxìshuǐ
  1. lit. mandarin ducks playing in the water
  2. fig. to make love

CEDICT

to have pure unadulterated love (of a romantic couple)

Wiktionary

Entry Composition

1 鸳鸯 yuānyāng
  1. (bird species of China) mandarin duck (Aix galericulata)
  2. (fig.) affectionate couple; happily married couple
2 戏水 xìshuǐ to play in water