i have a very big penis as i feel shy to open up to my girl friend
In a room full of 20 people, you'll probably find 30 different attitudes to this! I doubt there is any conclusive evidence, only opinion (and, like most opinions, it is affected by politics, upbringing, religious beliefs or non-religious beliefs, culture etc. etc.).
So, having said that, what's my opinion?
Like so many things, it's OK in sensible quantities, but not a good idea to over-do it or become dependent on it. It can be fun to watch with a partner, as well as helping reveal what turns your partner on so you can have better sex. It can give you ideas to try things you haven't before. It can help you explore and enjoy certain fetishes that are not practical to carry out yourself in real life. And it's fun, whether alone or with a partner.
I don't consider seeing pornography to be cheating - it's part of fantasy play. While society encourages fidelity to one person, it cannot be denied that, just because someone has committed to being with one person, it does not stop them finding themselves attracted to, turned on by, feeling desire for, other people. In fact, it's possible that porn helps satisfy that desire, and reduce the liklihood of cheating. Not sure about that, but it's possible.
HOWEVER, if you partner does not feel the same way, if they view it as cheating, you are unlikely to change their mind. So it's best to respect their views, even if you disagree, or your relationship could be badly damaged. If your partner isn't happy about you looking at porn, whether together or on your own, don't do it.
If you become too obsessed with porn, even if your partner doesn't have a problem with your viewing, it can damage your sexual relationship - if all your sexual energy goes into masturbating to porn, your desire for your partner may reduce, and you may find it more difficult to get an erection or to reach climax.
But all that is just my opinion...