I haven't had sex, so? I don't really need it. BLAM! I've got ways of keeping myself entertained. Like with my special, double-barreled 'sweetheart'. BOOM!

I think the point of that statement is not so much "Doomguy has not had sex" as "Doomguy may as well be a virgin because it is irrelevant whether he has had sex."

id Software has always been insistent that they value gameplay over plot. Things irrelevant to the gameplay or Doomguy's immediate situation – like, say, who he is fucking – may as well not exist at all.