It's both owners' faults in my opinion, but with more fault on the owner of the bitch in heat.
I understand when some of you say that people should be allowed to leave their intact female outside in a fence that keeps their female contained, etc -- "how can it be their fault if another dog jumps the fence?" I'm sorry, but this is the view of someone who wants to live in a perfect world. If you have an intact bitch, you have to realise that, while your dog may be perfectly behaved and stay in it's own yard, others may not. What if it was a stray dog that jumped your fence? Who do you have to blame then? Who will you be angry with? The male dog? No, it's your bitch, and it's your responsiblity to NOT ONLY make sure that you're keeping her in, but ALSO to make sure that you are keeping others out, and the only way to make sure you are doing this is to watch your female at ALL TIMES. If someone is naiive enough to own a bitch in heat and not understand that dogs will jump ten foot fences in order to get to her -- if someone is naiive enough to think that all owners of intact males keep their dogs perfectly confined -- IF the owner is naiive enough to think that there are no stray dogs in the world --- then I'm sorry, but this person did so little research and has so little commitment that they should not own a dog, period.
It's just like locking your toddler in a car and then someone steals your car. Yeah, the person shouln't have stole the car, but I'm sorry, you left your child in a vehicle. Yes, you locked the doors and had your child strapped in, so you protected your child from getting out -- but did you ever think that there was maybe a chance someone else could get IN? People are just as crafty as dogs when they know they want something -- and all dogs have to go on is instinct, which is more powerful than human's pure greed.
I don't care if either one of the dogs is a show quality, health certified, working titled, champion dog -- Dogs should not be left outside unnattended unless you KNOW that your dog can't get out, and someone else's dog can't get in. Occasionally I leave my dogs outside unnattended -- but I acknowledge the risks and I'm willing to take responsibility for anything that happens, and I NEVER leave my dog outside when she's in heat.
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Now, thinking of if it were me...
I have an intact female and an intact male.
If I left my female outside while she was in heat, and another dog jumped the fence and bred with her, it's my fault. I left her outside unnattended. I would talk to the owner, let them know that their dog can get out of it's yard, and let them know I'm displeased, but in the end it's my fault. By not paying attention to her I put her in the situation which allowed her to get pregnant. She's my dog, and my repsonsibility. I would abort. I don't care about the fertility risks involved --- I will not be responsible for bringing mixed breed puppies into this world.
If it were my male getting out --- legally, I would be at fault. But in my opinion, once again, while we'd both be at fault, the owner of the bitch was more at fault. If it weren't MY dog jumping the fence and getting her pregnant, it could be any dozens of other males, whether they are owned by someone or stray. So what difference does it make that is was my dog? It was bound to happen anyway! But yes, I should have been watching my dog -- I didn't know that my dog could jump the fence -- I didn't know that someone would let my dog out of the yard -- etc etc -- all things that I shouldn't even have to know, if I would have just watched my dog. I would offer to pay half of the abortion fee, if they refuse because they want puppies, I wouldn't have anything to do with them -- I wouldn't pay for anything. If they refuse because of religious beliefs, which is something that I certainly don't have but can understand, then I'd offer to pay half of any emergency procedures that may be required later on (c-section etc), and I'd offer to pay for half of the puppies' vet checks etc.