One thing I understood from my son, Izhak is that one ought not add to the commandments in regard to the chapters vayikra Leviticus 18 and 20 and that adding to these prohibitions means subtracting.

As regards to the actual subject--adultery means sex with a married woman. You can see this in vayikra Leviticus 18 and 20. Chronicles I chapter 2 verse 46

[See responsa of the Radvaz and Maharam of Egypt.--beside the already well known treatment of this in Rambam, Ramban, Raavad and in the Shulchan Aruch.]