And IMO, someone with an intact male dogs, needs to be aware that there may be females in heat in the neighboring area & keep their dog properly confined!
And since you felt the need to jump all over me, where did I ever say that it's ok to leave a bitch in heat unattended outside? I would never do it, so let's not go there. I've got intact dogs & have NEVER had an "oops" litter (or even an "oops" pregnancy for that matter). But that wasn't the op's question.
And let me ask this: We've got an 8 ft privacy fence, double fenced (meaning we had the ugly side with the beams showing so to be extra cautious--our OEB loves to bust his head through the boards of wood fences--we double fenced it. We also spent a lot of time & money digging a trench 1 1/5 ft. wide all the way around the yard under the fence & filled it in with concrete (as the neighbor's dog wouldn't quit digging under & we already had an "issue" with their previous dog who dug out a hole I had just filled in & got into my yard when I had a bunch of dogs out). We have electric fencing (horse & cattle) running along the bottom of the fence & along the top of the fence. We have 2 gates with heavy duty padlocks. PLUS, when I have multiple dogs outside, they are on tie-outs/chains. So again, after all the time & money I've spent "barracading" my dogs in my yard, if someone else's dog manages to get into my yard, whatever happens is 50% my fault? I don't freaking think so........................