There are universal definitions and laws whether you like it or not, and of these definitions is

A thing which does not harm you or harm others and at the dame time it helps you save your life and improves you or your life is called a good thing.

"No two cultures can agree on what is wrong or right, and I doubt any two people could either." is wrong a wrong statement for example  the Islamic culture or religion encourages marriage and prohobits relationships (affairs) out side marriage, and nowadays many organizations in USA fight to prevent affairs outside marriage, this is only one example.