I am seeing a guy while in a relationship?

[ESTRELLA]

Hey,
I am a 15 y/o girl and I am quite mature and responsible for my age.
I have a boyfriend (18) since 2 weeks ago, and we have been doing stuff since the end of november.
Now, I also met this guy (20) two weeks ago. He followed me on twitter for a year and then we started talking. He wanted to meet quite quickly, but I was okay with it and went to see him. At the end of the date he kissed me twice and ever since he has been in my head. He is so charming, even though when I saw him again yesterday he kissed me again, but when I told him I would rather not kiss because I have only seen him twice and I have a boyfriend, he told me he doesn't see the point of our dating then and acts like I'm a young child because I don't have sexual experience or desire.
What should I do?