Ibn Sirin (may Allah have mercy on him) said: I asked Abidah (may Allah have mercy on him) about this saying, "And you will never be able to be just between wives, even if you strive (to do so)" [Surah al-Nisa: 129]. He said: He pointed with his hand to his chest and said: (It is) in love and sexual intercourse.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب القسم والنشوز / 14742