A man who sees nothing wrong in sleeping with multiple women simultaneously, concealing one from the others without a hint of guilt. The term ‘multi’ suggests the progression from being a two-timer to engaging in more complex forms of infidelity. Women can't be multi-timers by nature of their gender that requires more fidelity: it's easier to call them "sluts" and move on.
Multi-timers are more akin to "manwhores" but slightly more honorable, as the latter are only into infidelity because they're lowlifes. Multi-timers just need more time to settle in their lives, so are looking for more choices in women. It's like when you browse the shelves of a supermarket, you don't really buy everything you see.
Randy is seeing Paula while making out with Molly, who's been sworn to secrecy about Paula. Molly readily obeys due to her own lack of moral compass. Randy also recently went to Las Vegas, indulging in threesomes with prostitutes. He's currently texting one for future encounters, all unbeknownst to Paula or Molly. Needless to say, neither of Randy's regular girls has a clue about his Vegas exploits.
Randy recently met an exchange student from the Czech Republic. They seemed to connect well, even making out at his office! Now smitten, Randy is planning a European vacation, possibly with Paula. While she enjoys Paris, Randy might travel to the Czech Republic to propose to his new girlfriend. This could lead him to unceremoniously drop his past girlfriends.
Basically, Randy is a multi timer.