Tl;dr: Not a simple answer. If you want a good answer you would probably have to talk with a lawyer in PA, but they will probably tell you it depends and not to risk it. Also, I am not a lawyer nor am I someone who studies law so please do not take my response as fact.
Probably the best explanation is:
“It is a big gray area,” Wright explains. Often, she says, “it depends on how the district attorney wants to interpret the prostitution laws and how they define sexual conduct.”
This is a quote in an article which can be found at:
http://www.nbcnews.com/id/27846842/ns/health-sexual_health/t/behave-toeing-legal-line-dominatrix/
This is the answer I would recommend. The pa law for prostitution reads:
A person is guilty of prostitution if he or she:
is an inmate of a house of prostitution or otherwise engages in sexual activity as a business; or
loiters in or within view of any public place for the purpose of being hired to engage in sexual activity.
Found at
https://statelaws.findlaw.com/pennsylvania-law/pennsylvania-prostitution-laws.html
With sexual activity being defined as Sexual contact is generally defined as intentional touching, either directly or through the clothing, of the genitalia, anus, groin, breast, inner thigh, or buttocks of any person. Definitions are primarily governed by state criminal laws, which vary by state.
Found at
https://definitions.uslegal.com/s/sexual-contact/
To paraphrase the earliest quote, it is probably legal, but it depends because law is complicated. There's the written law, and the interpretation of the law, you see. Additionally, being arrested for something and being convicted for that something are not necessarily the same thing. You may get arrested under suspicion or a misunderstanding, but cleared in (or out of) court when a legal expert(s) look at the case. Beyond that, there are businesses that do operate within PA regarding fetish activities. One might reasonably argue that because these locations are in business that the activities are safe and or legal.
Anyway, the take away should be that this is a grey area that isn't defined well. It is not explicitly legal or illegal. I think it reasonable to say you're probably ok, but act with the understanding that there is risk involved.
Also note that I didn't really pull up the actual statutes or laws, just locations that may have had them.
