If you are this uncomfortable with the situation, then why don't you sit down with your wife and talk to her. There is such a thing as addiction to porn and it is the same as with alcohol and drugs.
It could be that this is the only way she has learnt to express herself sexually. If the problem presists then try couples councelling or why don't you explore other avenues sexually.
Ask her what she wants, how she feels when she is watching porn. That emotion might be what she is trying to attain. Or she might simply be looking for other ways to make you feel good sexually.
Anyway, I am going on, so good luck!!!