I'm not trying to be obtuse, but literally don't understand how someone would get from "space ex" to "space sex" unless they are intentionally trying to stretch/double the consonant.
To be clear, I'm willing to agree that it's a funny mispronunciation, but am unwilling to accept that this particular naming was an intentional juvenile joke by Musk. It's widely known he's a fan of the letter X, and I believe that fully explains "SpaceX".