The letters "L" and "O" in the sequence of "L-O-L" has been over used on the internet for the past ten years that no one even notices them anymore. You type "Fuck you, lol." and the average internet user will read that and take from it that you're pissed off at them and seriously just said "Fuck you." To combat this, many have implemented an alternative method of showing their not serious in the form of "J-K," however much like it's predecessor, it too has become over used to the point that it not longer holds any value.
Though a side effect of the usage of "J-K" is one similar to that of "No Homo." It is often inserted at the end of a message that would otherwise not be question, but since you felt the need to put it on there, it indicates that for at least a moment, you were thinking of another man in a homosexual way. So when you say "J-K," half are going to skip right over it, while the other half are going to read it and view it as a back-pedal move. "I hate you, fucker (realizes what he has just said, and doesn't have the balls to stand behind it)... jk (don't e-kill me!)"