Another thread where everyone points fingers at the other side to theirs.
As always, life just isn't that clear cut and simple.
Neutering males has both positive and negative health and behavioural consequences so it is not right for everyone and every dog but may be right for some/many.
Even so, neutering a dog will not automatically stop his interest in females, will not auto stop him chasing after a bitch in heat and will not auto stop him mounting her. Moreover, you could go your whole life walking a male dog and never come across an in heat female (at least that you know about). Therefore, you would never know if your dog would remain "under control" around in heat females. Chopping his balls off or keeping him on a lead for his whole life, just in case, seems drastic to me (not withstanding there may be other reasons neutering is the right choice for you/yours or on lead walks suit you both better).
Bitches can continue to experience heats even after being spayed (as in mrsjoyful's dog's case). Restricting them to no walks at all throughout this period would surely depend on a combination how well the dog coped with not being walked, what other facilities you had for exercise (large paddock?) and the likelihood of you coming across a male on your walks. For example, a bitch who hates the cold and wet, who has access to a large garden and who lives somewhere where there are many dogs might do well to be kept indoors during a winter season. A bitch who loves her walks, has a small garden and who lives rurally where other dogs are more spread out and less of a risk may do just as well to keep them up throughout her season with little risk.
Even without this spaying also has both positive and negative health and behavioural consequences so it might not be right for all.
Surely, the best we can ALL do is make good efforts to train our dogs, try to make the best neutering/spaying decision we can based on the dogs we have, try to avoid situations that are obviously risky to them or others and try to be considerate of others. For me, that doesn't mean blanket rules that all dogs should be neutered or all bitches should be locked up during a season.
If all else fails, it's pretty hard to miss that your dog has been accidentally mated (if you are there with them) so a trip to the vet would be an option to ensure there are no unwanted puppies. kennelmaid has given a great example of why prompt action might be sensible.