We don't know why this pet duck was in the house and taken out on a leash. Wr don't know for how long _ou have had the other ducks. So don't beat yourself up about the situation now. It is as it is.
I rescue ducks and often have ducklings to raise and release again when they are mature enough. I need to keep them 'wild' with a healthy fear of humans, but can only do that when the ducklings have other ducklings of a similar size as ducks are flock animals. My first rescue was only 2 or 3 days old and needed rescusitating and intensive cate for his first 2 days with me. Once he recovered and was in the brooder, be became depressed despite having a soft toy duck and a mirror. I started searching on line for more ducklings to purchase to keep him company but the only ones I found were 4 hours away. I was not going to drive there and back with a sick duckling in a box next to me. So I had him out of the brooder during the day, on my desktop between me and my laptop so that I could keep him company and handle him frequently, while getting on with my work. That was for about a week until he would settle in the brooder on a stool next to my desk so he could see me whenever he looked up. By 4-5 weeks he was too big for the brooder and was moved to a dogcrate with lots of time running round my Florida room. I then found a rehabber who had 2 ducks about the same size -- one another muscovy likely to be similar age, and one a pekin with special needs that was 3 or 4 months older. All were likely to be drakes andso, I took in the 2 from the rehabber to be my first drake flock. My original rescue was tightly bonded with me, unsurprisingly, but all three were OK together and at 8 or so weeks moved out to a new duck coop in my garden. The original drake still follows me around, when he is in a good mood, still demands attention (by biting me) still likes to sit on my garden steps next to me being cuddled, and still tries to get into the house at every opportunity. But he lives in the duck house with the other, now larger number of, rescued drakes. Rescues come and sone are rehomed and leave. My special needs drake has passed away but I still have the muscovy that camefrom the rehabber.
If its posdible, your drake should live with the other ducks outside and just get attention frim you out there. No more going out on visits and no living in the house. It may not be possible, but do try it as it is more healthy for him to live with other ducks and to stop trying to mate with you.
Realistically he is imprinted on you, and you will remain the object of his affection unless he forms a pair bond with one of the duck hens. If he is 5 years old and hasn't done that so far, it's not likely to happen.
