I've had Labs for almost 40 years and multiple girls in my kennel over 30 years, and I've never had two that I had to keep separated when I was not with them- ever! Likewise for the boys, even when girls are in season. I do have two girls that play so roughly when I first let them out for their evening walk that I keep one on leash for the first minute or two until the excitement ebbs. I can turn them out into the airing yard (not as exciting as the daily walk) and they are fine together -best friends - no wrestling.
Had a Ch bitch with all clearances. She was a definite improvement over her mother in many ways except temperament.
She did not like other dogs! At all. Same sort of thing - started with the occasional growl and moved up to full out attack mode in a matter of weeks. She was good with our pack, it was dogs met while out walking, at shows etc. Her's is an insecurity issue - tried working through it, put obedience titles on her, etc. She was well socialized from a young age with people and other animals.
I did place her with full disclosure to anyone who inquired about her "intolerance" of other dogs. Loves people and cats, is a dream to work with in obedience and at the Vet etc. She lives with a wonderful couple who have two cats, live at the end of a dead end road and have no plans to add another dog ever. They love her to death and she boards with us up to 12 weeks each year as they love to travel. She grumbles at the dogs that might be new to our pack between our visits, but a reminder from me that it's not an appropriate manner and she's fine. But, for me it wasn't the Labrador temperament that is my goal for breeding.
So in all my recommendation for your question on how to approach her behavior when talking to prospective new owners is full disclosure.
