I think the line is crossed when there is real, unwanted pain or danger to health, life, etc. or if it affects others in a way they do not consent to.
I believe that there need to be inviolable lines that apply to us all and that line is anything that is a felony crime, anything that is done to anyone non-consensually, and anything (sexual or violent) done by an adult to a child, whether they consent or not, and finally, anything (sexual or violent) done to an animal.<br />
Then there is the personal line that each of us must determine for ourselves. For me that line eliminates anything having to do with urine, excrement, or blood and anything that inflicts serious pain on any party, whether consensual or not. There are many fetishes or paraphilias that have absolutely no attraction to me personally, many of which I cannot even understand, but that I do not condemn as long as done by consenting adults.
I do not know for me anything involving poo or wee or blood or dressing up as a baby but if it turns some people on then who are we to judge ?
I think when it hurts another person who is not someone completely willing, then it crosses that line. Do think there are many things that people desire that if two consenting results, want to try, should be able to, obviously there are some limitations with this but think at times, too many restrictions are in place. I think the key is both individuals being in consent and understand. Great question by the way
Hurt someone on purpose and enjoy it, you need help