He Says Cyber Space Infidelity Doesn't Count!

i've been with my boyfriend for four years. For the first year he was unfaithful with an ex mistress or two. When i found out i threastened to leave him and since then he promises that he's been faithful. However and I am not proud of this, i regularly check his emails and his internet history. I didnt take the first indications that he was using internet porn too seriously- after all many men do- don't they? But then i saw that these sites included teenage girls. I have three young daughters by a previous marriage; my boyfriend also has daughters. I am 44, my daughters are 14, 11 and 9. He is 61, his daughters are 26 and 23.
When confronted about this stuff he was chastened but the next horror was that he had registered on on-line sites such as fling.com or craiglist looking for casual sex. in one he even registered himself as "like it in the day" and said that though he loved his partner, "she is not all that i want". He claims that cyber flirting is a harmless way of remaining faithful while having some fun and that by checking up on him I making the situation worse. He says i should look at the good in our relationship rather than concentrate on the bad.
I think i am right to leave him.
I would love your advice.