The story I read said the other girls' parents knew about the relationship from the get-go, and the "gay thing" WAS the problem. They tried repeatedly to have Kate expelled from school, finally making that happen a few weeks before Kate was to graduate. So they got what they wanted, Kate had to change schools for the last few weeks of her high school career, but then as soon as she turned 18, they decided they wanted more and had her arrested. They are very much trying to ruin Kate's life, not because she is 18, but because she is a female that they believe "turned" their own daughter.
Also, I read that Kate was once called in to testify in a nearly identical case. She had walked into the school bathroom to find two girls (same age difference) engaged in a sexual act. The charges in this other case were dropped. Same school, same district, same situation...very different outcome. This could have led both Kate and her GF to believe, if something were to happen to them, the charges would be dropped as well.