Okay, my girlfriend's parents found out we had sex. They said they don't hate me, but I don't know how to explain to her dad how I'm sorry I disrespected him.
Disrespected him? It is your girlfriend whom you treated with disrespect. You used her for your own pleasure, even though she was willing to have sex with you. You did not protect her from yourself and from sin. Love "does not behave rudely, does not seek its own, is not provoked, thinks no evil" (I Corinthians 13:5). You did not treat her with love.
Nor did you treat yourself properly. "Flee sexual immorality. Every sin that a man does is outside the body, but he who commits sexual immorality sins against his own body" (I Corinthians 6:18).
Fornication, like other sins, will keep you out of heaven if you don't make the matter right with God. "Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God. And such were some of you. But you were washed, but you were sanctified, but you were justified in the name of the Lord Jesus and by the Spirit of our God" (I Corinthians 6:9-11). This is a matter you should be talking to God about (I John 1:9).
It is understandable that her parents are upset with you for not treating their daughter with the respect she should have gotten from you. They are probably disappointed that she did not follow God's teachings regarding sex as they brought her up to do. Yet, the topic that needs to be addressed is not your past sins, but what you are committed to doing in the future. The past can't be undone, but you can change the direction you are going from this point on.