It works in a bf-gf relationship, you are together, you are friends, you have sex, you become one...blah blah blah... but when you are only friends with somebody... it might put a wall between you...
Beside, how you can have sex with somebody who pi55ed you off... :)
A few of my Polish friends look for Polish girlfriends. Only Polish. Their explanation is cultural similarities. I tried to tell them to be more open minded, but it's a lost case. :)