I want to know if I can get married to my girlfriend. My dad slept with her. Is it possible for both of us to still marry?
The Law of Moses, which does not apply today, stated a son could not behave sexually with his father's wife (Leviticus 18:7) nor could a father behave sexually with his son's wife (Leviticus 18:15). Under that law, both situations resulted in the death penalty (Leviticus 20:11-12). These laws would not apply to your situation, even if they were still in effect, because you are not married to your girlfriend.
I'm assuming that your father is still married to your mother. As a result, he committed adultery with your girlfriend. Again, under the Law of Moses, that would have resulted in a death penalty (Deuteronomy 22:22).
Thus, there is no law, Old Testament or New Testament, that would directly forbid you from marrying your girlfriend. But there is an important question you need to consider: Why did your girlfriend commit adultery with your father? Do you want to marry a woman who is willing to break God's laws that state that sex belongs only between a married couple (Hebrews 13:4)? In other words, I would like you to consider what this sin indicates about the future if you marry her.
She did not plan to have sex with my father. My father forced her, in other words, he raped her. After that, she was crying even when he was having sex with her. She told me all that happened. I just loved her naturally. Even if she committed a sinful act, she can still be forgiven.
If your girlfriend was raped, then she did not commit any sin. The sin is completely your father's.
I hope you no longer have anything to do with your father until he repents of his sin. Even then, I would not ask your future wife or your future children to have to deal with him.