It is not okay to include others in your sexual activities without consent. That's the line. Getting aroused by doing something in the public / been seen doing something means involuntary participation of others, which is not okay.

Yes so thats the question, why do you not concent to someone enjoying without harming anyone. Lets say i ecstatically enjoy sunsets by the see. Should i be forbiden to enjoy them because i enjoy them a lot more than nearby standing people?