How do I get mom to understand that I don't need a therapist?

Im being punished for what I did, and I accept my punishment, but I don't really think that what I did was wrong.My dad's new girlfriend and her son moved into my dad's house about a year ago. My girlfriend's son is 15 and Ill be 14 in less than a month. We get along great most of the time. Bt I don't see him as a stepbrother or anything, our parent's aren't even married to each other and even if they were I still don't think it would change anything. Just because our parents live together and are dating doesn't really mean we are related. His mom saw us kiss in my room when she got home from work, and she completely overreacted. She started yelling at us and she started hitting me, so he did get her off of me and she stopped. But she was still freaking out when she told my dad, and everything went downhill form there. My dad says I betrayed him and that he will never forgive us. And my mom freaked out too, and she asked me why I did it and how far it went and she says its wrong. I told her what I thought and that I don't think its wrong, she kept telling me it is. I think I was wrong for going behind their backs, but I don't feel bad about the things ive done. That all happened yesterday, I ended up getting my phone, my iPad, my TV, and just about everything taken away and Im grounded for at least two months. And now she said I should talk to a therapist and "get things straightened out", but I don't need it because I know what I did and I don't need help. How can I get my mom to understand that I don't need a therapist?