love rat
noun [ C ] UK informal uk/ˈlʌv ˌræt/ us/ˈlʌv ˌræt/
（男性）偷情者，偷腥的人（该词常用于通俗报纸中）
a man who has had a secret sexual relationship with someone he is not married to or who is not his regular sexual partner. This word is usually used in popular newspapers.