Here is some advice on how to make it work: Don't sign on to this dude's brand of poly if it doesn't ring true to you. His ideas might not really work for who you are. Don't worry about turning him down, if you are really mono or want to take things slowly into poly-land, there are other people out there who might be better for you, believe me. He isn't the only fish in the sea, as they say, and his way of doing poly isn't the only way to do it - you may not want the hierarchy of having a "main relationship" with others on the side, maybe you want a couple equally important relationships where no one is secondary.
If you do sign on to do things his way, then don't just find someone else to have sex with because all of a sudden you have this agreement that says you can do that. If you can't imagine fucking someone other than your new BF, it's likely that jumping into something with someone else before you're ready will just make you feel like shit.
Wait until you meet someone you really dig, someone who makes you smile and feel good about yourself when you're around him or her, and THEN see if you want to move forward into something physical. In other words, put people first, not some idealized relationship configuration.
Oh, and make sure you have a discussion about safer sex practices, what protection you will both use, and how often you will get tested for STIs.
There is nothing wrong with being monogamous if that suits you better.