birchykel, It is probably the hormones that are making you feel even more insecure, and paranoid than you normaly would.
It took me a long time to get my head around it - the fact that he needed to pleasure himself on his own, without me.
I spent a long time dwelling on it, and was probably quite childish they way I acted towards him after that.
Every little argument I would mention it, and say ' oh go and have a wank then' or 'make sure you turn the porn off my computer'.
I realise I was un sympathetic, and I did make him feel dirty and upset him.
I also withdrew from sex a lot,and physical touching, which did make us grow apart slightly.
A few months after I had our first baby, I started to feel a bit more human again, and realised that I was making him suffer for something that isnt really that 'cheap and dirty'.
I also realised that if I carried on, I could risk driving him away to get his kicks from a real woman!!
We started just getting physical again by hugging, massage etc. Then once we got back into it, sex was just as good as before, and somehow I sort of forgot about him watching porn.
I also spoke to loads of friends, who knew there men were the same.
I dont actualy think he has done it since, although he says he does have the odd wank now and then.
So I think sod it, and tell him when ive had a fiddle if hes been at work!!
Whats good for one is good for the other eh!! 
coppernoddle is right, it will probably be one of the last things he does when baby come along, as he will be too busy with his new duties!!
You will understand soon, it just takes a while to realise you are never going to be the only woman in the world that he will find physicaly attractive.
Unless you buy him some blinkers
xx