Sex with a non-Jewish woman, what is the prohibition?

The Gemara in Avoda Zara (36) has a fascinating discussion about this, the bottom line seems to be the following:

1. There is no prohibition in the Torah for a Jewish man to have casual sex with a non-Jewish woman and vice versa. There is a machlokes tannaim and rishonim whether it is prohibited in the context of a marriage between them.
2. The Beis Din of Shem decreed that it is prohibited for a Jewish woman to have casual sex with a non-Jewish man because we are afraid that she will follow him and worship avoda zara. The obvious problem with this is that there were no Jews at the time of the Beis Din of Shem so how exactky did they make this gezera. 
3. The Beis Din of the Chashmonaim decreed that a Jewish man having sex with a non-Jewish woman violates 4 sins (Niddah, slave, married a non Jewish woman, slept with a married woman)
4. David's Beis Din prohibited yichud with an unmarried woman
5. Beis Hillel and Shammai prohibited yichud with a non-Jew

A few years ago it came out that there were frum Jewish men looking for frum Jewish women on Craigslist for causal encounters. What comes out from the above is that from a halachic viewpoint someone who wants to have casual sex is much better off with a non-Jewish woman even married then a married Jewish woman or even an unmarried Jewish woman. 

Sex with a married Jewish woman carries the death penalty for both parties, sex with an unmarried Jewish woman is almost always going to violate the issur of nidda which is an issur kares. On the other hand, a non-Jewish woman is never an issur d'oraysa, it is only an issur d'rabbanan.