Porn isnt all that bad providing it doesnt affect the relationship, thats the hard bit, does he even realise theres a problem?
I know its difficult but he perhaps has worries about the baby, about hurting you or the baby, or maybe just needed some "alone" time,
I know for a fact that since I got pregnant I myself have sorted myself out and likewise I dont doubt for a second my OH has too... more for the fact we're both knackered and a bit too lazy sometimes!

Yes its hurtfull but you have to decide if the hurt comes from the porn (was it degrading?) or the fact your feeling insecure and want him to pay you some attention and not the computer? Was it because he was hiding it from you?
Men have strange ways of dealing with things and it was possibly a comfort thing after the argument,
Talk to him about how you feel and encourage him to do the same,