Pesachim(112a): Do not cook in a pot that your fellow used for cooking. What is that referring to? – Do not marry a divorce during her first husband’s lifetime. As the Master said: A divorced man who marries a divorced woman – there are four entities present in the bed. Alternatively this advice also applies to not marrying a widow because not all fingers are equal [i.e., sex organs and she will think that sexual relations with her new husband are not as good as with the first and she will come to disparage him – Rashi].