Craigslist is an online "classified ads" site. People post stuff they want to sell or are looking to buy. It's broken down by the area you live in. There's also a very special part of Craigslist entitled "Casual Encounters," which is further broken down into Male looking for Female, Female looking for Male, Male looking for Male, Female looking for Female, etc etc. Gay men often frequent Craigslist, (though they aren't the only ones, just the most willing to talk about it, I think.) and use it just like they use Grindr... to hook up. That's the joke- Kurt is calling Sebastian a slut.
CW is an American TV station that is joked about for playing all the "pretty boy" shows. 90210, One Tree Hill, Supernatural, Smallville ect... All the guys are skinny, hot and all have the same hair. Excatly how Sebastian wears his.