I've got a large male dog, getting older now (10yrs) - still very sexually active, but he has been with me since he was turning 1 years. - We had two other female dogs in the household that would go into heat and he WOULD NOT leave me alone during that time. Two or three times a day if I was willing (I always was).
I'm glad you mentioned that. Later we got another inexperienced male dog, who would watch him mounting and mating with me, and he would copy him until he realized it also felt good. After that, it was a trading game, each of them would take their turns.