没有拆不散的情侣只有不努力的小三
[méi yŏu chāi bù sàn de qíng lǚ zhĭ yŏu bù nŭ lì de xiăo sān]
Translates into: 'There are no couples who cannot be separated; there are only third parties who do not try enough.' It addresses infidelity where third party refers to someone trying to break up an established couple due to personal motives.