What that guy meant was that slave route and love route don't happen at the same time. So if Tenzin fucked Korra in the slave route that doesnt mean it happened in the love route; they are 2 different things.
Now, we know that the sex DOES happen, Katara talks a lot about how she loved being our lover or fucktoy (love route or slave route), so I'm a fan of a theory that a guy mentioned here. It was something along the lines of: "every book follows a route so book 1 slave route after that book 2 slave route, etc. and the same happens with love route, first the book 1 love route and then the book 2 love route, etc."
It makes a lot of sense when you think about it.