Yeah, only dogs do the knotting, it refers to the bulb at the end of their dick that gets stuck in a female when they ejaculate.
Tips that helped me for the achievement:
Don't try it each round, buy as many anaphrodisiacs as you can, kill all enemies except for one dog, then mate with that dog repeatedly while taking the anaphrodisiac a second before you commence the sex. You can only hold 6 drugs at a time in your inventory, so I did it six times one round, went through another two rounds to buy the anaphrodisiac each round, then did two more matings in a single round. As far as I am aware, this takes a minimum of nine rounds taking into account the price and need to buy ammo. Also, be careful, because after a few times you lose your strength and it is a waste to boost it back up due to the cost. If you get raped by even a single zombie, you won't have the sexual stamina to get 8 knots. Same for if a dog gets you and you didn't take the anaphrodisiac.
Anyways, I don't think the dogs can get you pregnant, which would be cool to see in later updates if possible.