We live in a rented house, with the top and bottom floors split into seperate apartments. In the back is a huge yard for our dog (7 month black lab, male), and we just found out that the people who rented the bottom apartment have an 8 year old male dalmatian. When we first met them, they told us their dog likes to hump other dogs, kind of matter of fact like, as if there was nothing they could do about it. Sure enough, after a couple minutes of the dogs meeting, their dog repeated kept trying to mount our poor puppy. They eventually got the dogs seperated, but more or less I got the impression that this is going to be ongoing and they're not really concerned about it. They also don't leash their dog much anymore, he seems to mainly run wild though responds to their commands, sans while humping.
Both dogs are males and fixed, and at only 7 months, Link is almost as big as their dalmation. I'm hoping in a few months he'll be big enough to fend off the older dog, but in the meantime is this something I need to worry about? I tried to sort of hint that we hope they don't keep letting this happen, but we didn't want to make a huge stink with the new neighbors we'll be having. Am I over-reacting, or should I be doing something more to prevent this? We'd like them to be able to play together, but not if my dog is going to get ummmm.... violated each time

He's only a puppy after all.
Thanks for any advice.