wow that a tough one huh alex??.....lol...well another guess here but yea his mouth is there and fluids are being exchanged from vagina to his saliva in his mouth , which spreads around the whole mouth......
let me ask....did he go to the doctor?....did he ask how is possible??.....
Total guess, but the tongue is on contact (at least part of the time) with most of your oral cavity. If the tongue is infected the infection is in contact with other areas susceptible to infection.