There is really no justification for cheating on somebody and blaming somebody for your cheating is pretty ignorant. That being said there isnt enough info here about you or this guy to say that you couldnt have prevented it, whats the old saying you lay down with dogs and your gonna get fleas. If the dude was a "scumbag" you shouldnt be suprised, shoot you might should have of expected it, if that is indeed the case.
This may not pertain to you b/c this doesnt seem like it bothers you much, if any. But one of the most annoying things to me that I see all the time are these girls who piss and moan about their guy troubles and how they treat them bad(cheat, lie, general mistreatment) yet still go after the same type of guy. So to say its your fault is wrong to say that you may be to blame not for his cheating but for you being in the situation might be accurate. Hope that helps