[NSFW!!] Is it unethical, or illegal to sexually exploit a young woman with an IQ in the range of 52 to 55?

[NSFW!!] When I was back in high school, a distant friend (15ish) of a female cousin mentally challenged to some degree (IQ around 52–55). She was functional but not quick to reason things out. My cousin told me her parents had to put her on birth control because the neighborhood boys were taking terns being ”her boyfriend”. They knew if they told her that she was pretty and they wanted her to be their girlfriend, they can play her into giving them sex. Then when one young man would get his fill of her, he would dump her and one of his buddies would take up being her ”new boy friend”. Seeing they did not force her into it sex with them, more like bamboozled her, is it illegal for those young men to play her like that or was it just unethical?