Yep, keeping mix-sexed, intact dogs in one household without an “oops” litter is 95% planning and 5% luck.
Even with crates and walls and doors between them, I do not allow my intact animals to be alone in the house together when a bitch is in season. Seen belgians literally chew through doors and wire crates from boredom, I don’t doubt for a minute that they’d manage somehow with enough hormones onboard.
That kind of awareness, dedication, and WORK is not reasonable to expect from 99% of the pet owning public, the folks who think a couple walks a week and knowing how to sit is more than enough exercise & training for Muffin. So yeah, I’m firmly in the “fix everything coming through rescue and pet contracts on most every puppy” camp because Joe Public, even with the best intentions, is not likely to be responsible enough to keep intact dogs.