I understand the feeling of infidelity considering he's been keeping it a secret from you, but it really is essential to remember that watching porn is an incredibly detached act. It doesn't say anything about you or his feelings.
Do you know why he might be watching so much of it? Maybe you should reconsider your sex life? (Not making any judgements, just throwing ideas out)
If it bothers you that much I'd speculate it's probably a big self-confidence issue on your part, in which case I'd urge you to try and address that first and foremost. Good luck, hope things get better