when you talk about a friend or a lover, people don't seem to know the difference.well for me i think they are quite different,because they are limit to friendship why?though a friend can still make those sacrifices that people feel only lovers can make , but you cant have sex with your friend, because if you call somebody your friend you should have the agape kind of love for the person,then the one a lover can have for the opposite lover is call the erotic kind of love.both are the same thing with just one difference.that is why the word relationship can be used for both friends and lovers but the word affair is restricted to only lovers.

I'm in complete agreement. You don't shag a friend.