Being a Christian doesn't remove temptation. Not only is it simply wise to avoid any situation where you could be tempted, it is also smart to avoid any situation where a credible accusation could be made. Even if you would never be otherwise tempted, do you really know the other person isn't? Or do you know for a fact their SO or yours isn't going to jump to the wrong conclusion?
"I know what I am doing" -- famous last words.
I find this whole premise that men cannot be just friends with women as quite insulting to men. Not all men are slaves to their sexual desires, and there are many things in this life and in the life to come that are more important than sex.