The primary reason for this is that while the Spartans were literally having sex with one another all the damn time, they actually took the whole 'war' thing seriously, and are remembered primarily for their fighting acumen, and secondarily for their child molestation.
Compare this to the main characters from the game Army of Two:
While I'm not saying that writing the word 'Merc' across the codpiece of your body armour is necessarily an advertisement for your services as a male prostitute, it certainly makes a persuasive impression in that direction, and I haven't seen anything in the game to suggest another interpretation.