Suggestion overall is to look for a dog breed that will suit your lifestyle around all things dog, not just sex. Starting off with knot size, rough sex traits in the conversation is kinda meh, and is why you got the reactions you got off the bat.
Golden retriever is worth considering. Good energy still needs a lot of play as you mention, open field fetch, hikes, good walks, etc. Super mellow and chill, friendly to everyone and other dogs. Easily adapts to your lifestyle, if you want to go on multi mile hikes per day , or just a few walks around the neighborhood he will adapt. You may wind up with a larger dog than 40lbs, like 45-65 for the middle-ground, but you can get larger and smaller depending on the traits the breeder was going for, and if it is an american, canadian, or english line. Also consider there are field and family dog bred traits that breeders will go for. The field bred lines are going to be smaller and more active and will need a job that simulates hunting/retrieving activities to be happy. If you don't plan on that, I'd go with a more family dog/support animal bred line that has been bred to be more chill people-pleasers.
Look around the forum, many will say that if the cards fall right and he grows up to enjoy human intimacy they are highly capable sexual partners and lovers. Definitely the males will be adequately equipped for human sports. I have no opinion if they are going to fit your other desires like snarling during sex etc. I feel those are stretch goals and will be random no matter the breed.