Oh.. my.. God...
That's just insane. "Feel free to enjoy the park - just make sure to reproduce first"
If you're a parent observing people without kids behaving oddly,
then call the cops, let them have a talk with the person.
Speaking of calling the cops, I had to do that once, unfortunately I didn't have a clear enough head to do things in a better order:
Me and the kid (yay, I'm allowed!

) were in a small park in town that has this little "pool" perfect for wading in. He was there to wade, I was there to eat strawberries. Other kids looked like they were there to pee in the water, but that's not the subject here. So, lil' todlers in just undies or swimwear, one woman let her lil' one in with nothing on. Not an unusual sight on beaches around here, but we're in town..
Anyways, this slightly scruffy looking guy is sitting a couple of benches away, noone else sitting on "our" side. He has his sweater in his lap, and I keep my eye on him. I notice movement under the sweater, and after a few seconds I catch a glimpse.. So, I get up (slightly boiling), go over to him, lift the sweater, point to the camera and ask what the hell he's up to? Then I tell him to get the hell away from there. He gets up and walks away quickly without a word. I calm myself for a seconds, and
then I call the cops, give them a description and tell them which way he was heading.
What I should have done was just call the cops straight away, keep my eye on him, and let them catch him sitting there with the camera.