The term 'adulterer' always implies that the person is married or in a formal partnership. The unmarried man was not an adulterer, even though he dated a married woman.
Pro Tip 2/3
Negative Connotation
'Adulterer' carries a strong negative judgment and can be considered offensive. Calling someone an adulterer can tarnish their reputation severely.
Pro Tip 3/3
Cultural Sensitivity
Views on adultery vary by culture; some may find the term 'adulterer' more stigmatizing than others. In some societies, being labeled an adulterer can lead to social ostracism.