My dh has it and swears blind it is not sexually transmitted. (Bless him) he says he had his first outbreak when he was 21 and with a gf for a year. He said he never strayed so it can't be. (a little naiive I think as his gf probably gave it to him and kept quiet!)
Just being nosey as it's a bit of a non issue as we have finished having kids now but is it DEFINITELY always a STI ?