Don't try to justify your actions; you're encouraging illegal activity and contributing to the exploitation of vulnerable persons.
What ineligible said is irrelevant to your argument; he said nothing of STI/STD's.
Also, you've sunk as low (actually, LOWER) than your 'girl friend' has. So she slept with a couple guys; that doesn't give you licence to seek revenge, nor does it justify your actions! It's cheating regardless. Are you so morally bankrupt that you believe that one wrong justifies another?
Follow the advice of Ej4 said; your relationship has been dead for a long time; move on.