Why does the girl I like who you think likes me seem all like a good girl and nice but has sexual contact with my friend?

Because she NOT a good girl; or she's an independent thinker; or she's needy; or easily persuaded; or anything you might imagine. The real question is why do you believe that you like this girl if you don't believe in her, don't respect her, and sound like you don't actually like her as a person. Could your interest in her be sexual; perhaps your attraction to this girl is no different than what you are imagining she's doing with your friend. Think about it.