Bill and coo
Definition 1 of 1
Pro Tip 1/3Romantic Gesture
Use 'bill and coo' to describe affectionate, romantic actions, especially between couples. 
At the park, many couples sat on benches, billing and cooing.
Pro Tip 2/3Not Literal
'Bill and coo' is metaphorical, referring to human affection not actual bird behavior. 
They're so in love, always billing and cooing, not unlike lovebirds.
Pro Tip 3/3Positive Imagery
The phrase creates a warm, positive image of people being close and loving. 
At the reunion, the elderly couple billed and cooed like young sweethearts.