What does it mean to "sexualize" someone

I was told that cause I accidentally called a 17 year old woman that she is hot, which I mostly did just to be nice
that I was "sexualizing" a child. I barely looked at her pics and we have talked since I apologized to her for any discomfort I caused her and we are cool. I'd definitely wasn't burning in my lust.

I just want to know what that means cause basically I was just trying to tell her she's not ugly.