trans | friend's question

I tend to have problems answering questions when put on the spot and I tend to have problems explaining things so I figured I'd jsut post it here and see what other people could come up with and pass that on to my friend.
"Where do you draw the line between a very effiminate guy/ masculine girl, and a transsexual? Why not just be happy being an effiminate guy/masculine girl?" Thanks in advance for giving some answers to these questions.