To be perfectly honest, i think you are over reacting.
If he was watching porn all the time, and neglecting your needs, or if the porn is having a side effect in your relationship, ie, he's needs porn to *function* i'd understand...but it seems to me like he was fine with you till you saw pics on his computer, In other words, his behaviour to you hadn't changed regardless of him watching porn behind your back.
Firstly, porn aint another woman, he wouldnt suddenly fall in love with a porn film and want to end your relationship...No matter how many pics of a (fantasy) porn star he has, he has only one woman, you, in his home.
Secondly,From what i know, most men like to watch porn once in a while. Its just porn. Its a bit harsh to expect him to go from porn lover to NEVER seeing porn again. Most men i know (if they love you) would do exactly what he's done in that situation ie promise they wont do it again then try and be more crafty, which is more dangerous IMO
As for him lying to you, i dont think its a good idea to force anyone not to do something they like to do.
This is the reason why he is lying to you.
If you really love him and want him to stop it, i suggest you drop the ultimatums, (he will keep doing it behind ur back if you keep that up, hun), try and stop being judgemental and find out just how into porn he is, then work on reducing it.
If you're not prepared to work on your relationship, (and bear in mind that all relationships are hard work) he is probably right.... you dont love him very much. You should leave him and get someone else.
JMHO