i know sex is natural but my 15 yo daughter had sex and i can't handle it, it occupies my every thought.
It's natural so it's an amazing time to you became a very good friend in that matter and protect your child. Don't fight and if you guys already had some stress about it just try to talk again and don't let it be a block between you and your daughter.
She is the most precious thing in your life and you want her healthy, protected and not being pregnant before time so your friendship with love will help a lot.
She will trust you... open yourself because you can't change it and been angry can traumatize your baby girl that sex it's not healthy.
Since she is sexual active just understand that 70% of the parents that live all around the world has a 15 years old daughter that has lost their virginity too.
Talk to her and say that you don't think that sex is bad but you should have it with someone who cares about her and to her watch out the guys who will just search for sex and she doesn't exposure herself to be emotionally used by those guys but if she is in passion for some guy and he is just searching for sex. You need to understand that if that moment was nice to her this is what matters! she is a young woman who needs a very good support so she doesn't feel rejected by you! bring her to the doctor and let the doctor talk to her about protection and buy some books for her.
Make questions like... how it felt? if she was in love for the guy that she choose and you can tell that you got angry because you don't want her to get hurt because she is precious for you.
Many times our children search for emotional connection because they don't have this close relationship with their parents so be there for her and use it to open a new place for both of you to grow in love.
Talk to her how was your first time! and what you felt so you can open her to talk to you.
70% of the girls lose their virginity between 14 to 16 so just be supportive and be her friend. If you say NOT!! it will happen anyway and it will be without your support and also without your daughter's friendship and she will be less protected and you won't know what it's going on.