From a Thesaurus:

Noun 1.        rounder - a dissolute person; usually a man who is morally unrestrained

*debauchee, libertine
*adulterer, fornicator - someone who commits adultery or fornication
*bad person - a person who does harm to others
*gigolo - a man who has sex with and is supported by a woman
*profligate, rake, rakehell, rip, roue, blood - a dissolute man in   fashionable society
*ladies' man, lady killer, seducer - a man who takes advantage of women
*swinger, tramp - a person who engages freely in promiscuous sex
*debaucher, ravisher, violator - someone who assaults others sexually
*philanderer, womaniser, womanizer - a man who likes many women and has short sexual relationships with them