Porns are simply cheap trills which 'may' bares more harm than
educational messages. So watch it with the right mindset as many
porns are simply an act of some sick fantasies.. i.e. rape, mind
control, etc...
Oh.. Porns can be as addictive as drugs.
Regardless, it is pretty normal for guys at your age to be curious
about such things. Problem is, girls at your age may feel that this
is a flirty behaviour and more or less, conservative. This is
actually a good thing as girls who are less conservative and
careless.. often gets into troubles.
As for what to tell her? Well, just be honest loh. Admit that you
are watching it or otherwise you probably is not normal. However,
should she disliked this behaviour of yours, well.. it's up to you
to convince and gain her trust. Trust needs hard work and time to
build. So.. good luck to you. 
