What should i do to find out weather i am gay or straight?

i don't know , what i am.....i mean um gay or bi? this ques irritates me a lot. i am having a gf and we are happy. but i likes to check out guys too. i always stare at guys and kind of attracted towards them and when i meet some guy then there are lot of problem's occur between me and my gf. i love her very much. what should i do?