What is the right time to have sex?

I recently found out my sister (16) had sex with her boyfriend and not just once 12 times.... im confused because she doesn’t seem like that type of girl at all. She didn’t even tell me she did untill 9 months later. It really hurts for some reason. Should i be freaking out or is it normal. I hate society today. I thought sex was for marriage. Its nice to see the real word... it sucks. She didnt even feel bad for not telling me...She smirked and said it felt good... whata *****?