Elliot and his daughter s1 question

Sorry if I’m soooo insanely off about this tiny plot line but I was watching a tiktok and the scene “dad I’m a virgin” with stabler and his daughter came up and it brought back old questions I had…. Sorry if my memory of the plot is choppy.

Why was it such a big deal his daughter learned what sex was or how babies were made and why’d she said she was a virgin 😭😭😭 I never understood the big deal like why was she so upset with him