What is a word for a man who has a lot of sexual relationships?

Solution 1:

I suggest Lothario defined by Wikipedia defined by as an unscrupulous seducer of women.

It's from the name of such a character in Nicholas Rowe's 1703 tragedy The Fair Penitent

dictionary.com adds the definitions rake, libertine and synonyms Romeo, Don Juan, Casanova.

The Office Romeo is a well-worn term, but I don't often hear Romeo on its own.

To my ear, Don Juan and Casanova have become rather quaint 'literary' clichés much overused by the mass-media, but somehow Lothario still seems to have a certain freshness. Maybe using it gives the impression you're well-read, rather than getting all your words from popular tv dramas.

Solution 2:

It is often referred to as a Casanova after the Venetian womaniser.

Also related, but actually predating the real person Giacomo Casanova is the fictional character Don Juan who has inspired among other masterpieces the Opera "Don Giovanni" by Mozart and the play "Dom Juan" by Molière.

Wikipedia pleasantly notes that Giacomo Casanova probably attended the première of Mozart's opera in Prague.

Solution 3:

Promiscuous - not being restricted to just one sexual partner. It is gender-neutral, so you'll still have to specify the sex of the subject.

Solution 4:

philanderer - To engage in many love affairs, especially with a frivolous or casual attitude. Used of a man.

Solution 5:

Satyromaniac [sey-tuh-roh-mey-nee-ak]:

a lascivious man; lecher

It derives from Greek (sátyro) for the mythical woodland creature that was well-known for its debauchery. The female equivalent is far more recognizable, nymphomaniac.