Most Helpful Guy
I feel sorry for you *internet hugs* lol. Anyway it's mainly psychological. I learnt in A-level Psychology that when someone does something or feels bad about themselves for instance cheating they project there feelings onto the other person. E. g. If someone feels guilty for cheating for example. They subconsciously suggest that you caused them to cheat to protect themselves from the unconscious part of the brain which is where the honest truth is located in the brain. He reacts like that to blame you for his behavior and it's his coping mechanism for dealing with the guilt of cheating. He's suppressing his problems and for him to cope mentally he finds ways of making you at fault. So he's probably thinking in his mind "ohh she didn't give me enough bjs" or she didn't want sex as much so she's selfish. Random things like this.
The human brain has 5 defence mechanisms this one is called Projection.
Most Helpful Girl
It seems to me, @andreah, that he got Caught Red handed and became Red faced as well and as Hell, with his hand in someone else's cookie jar and is Feeling the Hate because he Blames You in his own Guilty ridden way for Having to go out And... Cheat with an absolutely trashy girl.
You both may have been having some problems where you were Airing some dirty laundry at home and along with the dog's poop you Now have to Scoop up because of what he has done, it caused him to 'Do the dirty' by being this Cheat Sheet, whom I am sure you know Now, you can never trust as far as you can throw, not even to dump the garbage.
She may have had some 'Less attractive' Qualities that he had found Enlightening, and even though in the midst of 'Working it out,' he could have been yapping to this stray and you never even knew it, who knows. It's obvious he wanted his cake and eat it Two.
Now it seems that he is finding the Cowardly lion way out by not only going 'Ballistic,' and Changing this and Blocking That, he is Sticking it to you by Handing More of his Dirty works over to you with his own Dogs, who Unlike him... Are Not in the dog house and don't deserve to have this taken and twisted up their own tails.
Ditch him and don't allow him to dig up anymore bones for you. Bury him, give him his walking papers, his little doggie has barked up the wrong tree and will always end Up... Throwing you to the dogs every time.
Good luck. xx