Okay, firstly, it is fairly normal for people to want to look at porn and masturbate while they are in sexual relationships-- often this is a way that you can explore fantasy that your partner would not be comfortable with, or, for instance some people do it as a way to relieve tension when they don't feel up to giving their partner the attention that they deserve during sexual activity (since partnered sexual activity should be a two-way street, after all). There are other reasons too, but to know your partner's reasons, you'd need to ask him.
What is an issue here, that I see, is that your parnter lied to you-- you need to confront him about this, and work out why. Possibly, its just that he felt embarrassed, and it could make your relationship a whole lot healthier to have it out in the open. (If your partner has a history of lying to you, however, then perhaps its time to demand that he sort out his honesty issues if he wants the relationship to continue.)
There could be other issues-- for instance, you might perhaps have problems with the representation of women in this porn. There is porn available that represents women in a positive way (largely thanks to our own Miz Scarlet and those who have followed in her footsteps), so if this is a problem, and if you are both over 18, you could try finding porn together in which all people are represented in a cool and healthy way.
Also, take a look at this Scarleteen article, all about porn:
Looking, Lusting and Learning: A Straightforward Look at Pornography
[This message has been edited by Beppie (edited 09-29-2005).]