That's very sad and lame. How's a man know how to relate to women and be kind to them, if he can't be a boy who plays with them? Obviously, my children can play with whoever they make a connection with on the playground. I'm around and keeping an eye to make sure they're safe. I have seen some things from other parents that made me think "Wow" before.....as a mother of little boys, I'm more sensitive to the sexist mess society tries to stick on boys. It's just sad.
I would have just said "Aww, they were playing nicely, I just thought they could get hurt if they went down the slide at the same time"....because I wouldn't want the boy getting fussed at for playing with my kid (and something I said), and I wouldn't want her thinking I was being passive-aggressive like I was trying to say "stay away from that boy" or something. Then it could be in the mom's court if she actually wants to reiterate that she doesn't want her son to play with girls. If so, she's a nut job and that's entirely on her.
I had a neighbor once who was weird like that with her daughter. The daughter was 6, and a boy I babysat was 6, and he thought she was so pretty (because she was). We would play kickball or army dodgeball together and she'd join in (until her mom said she couldn't). One day while the little ones were napping, I pulled out sheets of paper and paints, and I painted with the 6 year old boy. I said "Let's paint things we like to do for fun" and I painted a girl reading a book at the beach. He painted 2 little boys (him and his brother), a black dog, and a little girl (the neighbor) playing soccer. I thought it was adorable. He was leaving and saw her outside, so he gave it to her. The mom seemed really miffed about it, which was weird. Like, she didn't want boys playing or talking to her kid.....but this was a very good, polite boy...as good as anyone could hope to babysit for. It was very strange. Can't help stupid, I suppose.