Porn - Your thoughts

What are your thoughts on porn? 
I've always heard- evil thoughts - manifest to evil words only to inevitably turn into evil deeds... 
Do u think that watching porn - (secretly) adds to intimacy in a relationship- or does it hinder it? 
If it was thought to be ok- why would someone hide it? 
Furthermore do u think porn is gateway to infidelity? Please reply. I'd like to hear everyone's perspective.