My Dad Is Overreacting. Why doesn't he want me to date?

I'm a 15 year old girl and I've been with my boyfriend for about 2 months now. I recently told my mom about it and she was okay with it. So about 3 weeks ago my boyfriend was walking me home and I noticed my stepdads car was outside of my house. My boyfriend was about to go home so he kissed me and as I was walking home, my stepdads car was gone. I freaked out because I hadn't yet told my parents that we were together. A few days ago my mom threw me a party for my 15th birthday and my biological father (lets call him Nick) attended as well as my stepdad (lets call him George). My boyfriend couldn't make it to the party because it was his sisters birthday. I walked by Nick and George and I overheard George tell Nick that he was telling him this because they have both raised me and he is also a parental figure in my life. I am pretty sure George told Nick that he saw me kiss some boy as I was walking home. Now Nick is a very religious person (he's Catholic if that helps) and I have never told him about any guys I've dated because I respect him very much because he is such a calm and friendly person. I am his only child. I remember him speaking to a group of young people at church and he said the purpose of dating is to get married (I agree but I don't think it has to be right away) so there's no point in dating at our age. I've never really talked about dating with George because he was absent in my life for about three years when him and my were having problems. However, my mom just says she doesn't want me to waste my time. Last night I went out to eat with my family and my dad told me after, "I'm watching you. I know things that you don't know about. Behave." and that's when I knew he had been told. I really like this guy and I don't want my dad to mess it all up. I feel that the next time he has me alone he's going to bring it up and somehow make me feel really guilty. What do I do?