YABU because it wasn't porn and digging out Kama Sutra and other sexual related articles is what teenagers do. I was the first generation with computers with internet and the very beginning of webcams so you would be shocked to know what teens do on internet (much worse than reading a book about sex).
She is a teenager and she is curious, I am surprised you haven't had a chat about sex with her actually because the worst she can do is learning sex from porn (where women are objectified and treated appallingly just to please men).
My youngest sibling is 12, at 11 he brought me a book that talks about puberty, sex and everything in between and we spent a few nights reading it, I wouldn't show him porn (again because porn is not where I would want him to learn about sex), but I talk to him openly about sex, consent, period, anything that answers his questions but also prepare him to have an healthy sex life and to treat his future partner appropriately when he does chose to have sex for the first time. I want him to know he can come to me with any question but I mostly wanted him to know that porn isn't real life and that while I know he will check it out eventually (if he hasn't already) that it doesn't trump real advice from "real people" especially women who are going to be his primary partners (I assume).
If I had a teenage sister/daughter I would want to have this type of talk even more, teenage girls are so pressured to have sex even when they aren't ready or don't want to, and to please men and to not let their needs known or ask for them to be met that I would find it my duty to have a chat with her about consent, pleasure, porn etc...
I am in my 20's and I am shocked by the number of female friends I have who are used to going with the flow and end up having sex when they don't feel like it because their partners nag for long enough etc.., because nobody has taken the time to have a serious chat about sex and what's appropriate. Instead of telling her off about reading the Kama Sutra I would more want to talk to her about whether or not she was looking at it out of genuine curiosity or fear of being a bad "performer" because I would want to discuss that with her.
Your husband is right, by making sex, which is perfectly normal and acceptable activity even for teenagers to take part in, taboo, you are forcing your daughter to look for info on the topic in your back and potentially come across misguiding information/bad content (unethical porn) and likely stop her from feeling she can ask you questions or come to you with her doubts which is the opposite of what you want because you want a daughter who is aware of sex and sexual dynamics and know she can come to you if she has any questions or feel unsure about certain things sex-related.
Being strict about sex has never stopped teenagers from having sex, showing your daughter she can come to you at any time regarding any topic might save her from making a few mistakes or ending up in certain situations.