Boy did you pick the right place to ask this! Here are some links to places that have bought my writing - remember to write a good story before you write the erotic part!
You are gay if you are primarily sexually attracted to members of the same sex. I would wonder why a straight person would watch gay porn? but this does not mean that they are gay.
No. Sexuality is pre-defined at birth, watching gay porn will not cause you to become gay, although it may cause you to be either more wary of homosexuals or more accepting of them.
Liking gay porn doesn't mean your a bad person. Many people who like it aren't homosexual. Maybe it's just curiosity. I think this is normal... remember, nobody can ever tell you what to like in...