He wasn't speaking about the act of lovemaking, but rather trademark patents on the company's next vehicle. Tesla already has Model S, which they are selling at rates faster than production, and Model X, an SUV crossover that's in development. The third model was going to be Model E, making it Models S.E.X.
Unfortunately, Ford ruined the punchline of the joke. They already have trademarks filed for the Model E dating back to 2000. Tesla tried to file for the trademark, but quit when Ford called and threatened to sue.
"In response to a request from Tesla for Ford's assistance with a trademark issue, Ford and Tesla entered into a contract in 2010 in which, among other things, Tesla agreed not to register or use Model E. When Tesla later sought to register the Model E trademark, Ford insisted that Tesla abide by the parties' earlier agreement. The matter has been resolved amicably," said a Ford spokesperson.
All good jokes must come to an end. Musk also told investors that he will remain CEO of Tesla for four to five more years, which will see him through the production of the third model.
"Nobody is CEO of a company forever -- eventually they carry you out," he said.
Musk is also founder and CEO of the rocket company SpaceX.