I just started telling partners years ago that this was what I was into - this turns me on. Before the internet, I never had the courage to tell anyone. But after joining several of the heart / stething related groups back in the 90's, and reading up on what other people are into, I came to the conclusion that what we are into here is really pretty tame and would probably be well received by someone who really is into you as a person.
I remember back in the mid 90's the first gal I told about it had a mom who was a nurse. Turns out that when she was little, her mom gave her old stethoscope to her to play with when she bought a new one. She went back to her mom's house and found it in her old bedroom and brought it over for us to play with that weekend. She couldn't believe how much it turned me on. Because I was so into it, she also got into it.
I have told every woman I have dated since, and almost all of them have really gotten to like it after they tried it. I have been married now for 8 years, and my wife loves it, probably as much as I do now. And she had no idea that stething / playing doctor was even a "thing" before I brought it up. So people can be "converted" to like this.
I would advise the single people to not go out trying to find a "steth partner" who is already into this - as it is so rare. Simply find someone who you think you can be happy with - someone loving, caring, accepting, cool, trustworthy, someone who you find sexy / attractive and share other interests with as well, and after you have been on a few dates with them and it is going well, tell him / her about it and if they like you, they should be open to at least trying this. After all, it's not like you're asking to pee on them or give them an enema, or tie them up and beat them.
Those things I could see some heavy resistance to (read: run the other way, FAST!) but this is harmless and fun. I don't know why anyone would object to having their heart listened to during a makeout session, a play doctor session, or foreplay or sex.
The only caveat to this would be if someone is still in high school. I wouldn't have ever told anyone in high school about this, because the rumor mill is so active and kids are so mean and like to tease each other so much. High school kids have no remorse and they tease each other relentlessly. If someone found out your fetish in high school and told everyone, you'd likely never live it down and I could actually see this being something that would drive a weak or mentally ill person to suicide.
But after high school, no adults in the real world really give a crap what you are into, and even if they heard about it they would probably just shrug and move on without giving it a second thought.