我们只是在玩的时候动了心
[wŏ men zhĭ shì zài wán de shí hòu dòng le xīn]
It says 'We just got attached when playing'. This could reflect a situation where casual interactions unexpectedly develop into true affection, usually used in a somewhat sarcastic way indicating that the result was unintended or regrettable. In short, it suggests falling in love by accident.