Taking Out the Trash
Let's consider your options.
You could kill that boy next door. This is the most efficient and fast-acting solution. It allows you to keep your apartment while improving the view. If we were still cavemen, either you would have clubbed him to death by now, or he you, but the situation would have been resolved without anxiety. Unfortunately for you, murder is frowned upon in your neighborhood, and is nationally reserved as a political tool used mostly overseas.
You could move. Have you considered nesting in Brooklyn's Park Slope amid the lesbian legions? On Craigslist, I found you a 1000 acre ranch in North Dakota for the price of a 300 square foot studio in your neighborhood. Your closest neighbor would be invisible without binoculars. But there is the matter of the cattle. How can you be sure that a randy steer might not horn in on your man's affection?
I am reminded of what the Mother Superior says to Maria who had fled the ardors of Mr. Von Trapp and returned to the safety of the abbey.
You could confront that boy-next-door. You could sweetly tell him that you are aware of his intentions, and that if he ever touches your man, you will lock him up in the basement of your building and torture him for three weeks before letting him die.
You could hire somebody to set him up, get embarrassing photos and use them to blackmail him. This might force him to leave town. (This once happened on the Young and the Restless, keeping Jack Abbott and Jill Foster Abbott terrified for more than a year.)
I hope by now you'll realize the absurdity of any of these courses of action.
There really are only two things that I think you can do that will demonstrate that you are a gentleman.
First, you can say and do nothing, trusting your partner when he says that you have nothing to worry about. It's what Jackie Kennedy did. It's what Princess Diana could not do.
Or, if you really can't force yourself to do that, find an opportunity when your man and the boy-next-door and some friends are all present. (Maybe at an impromptu encounter on the street, maybe at a bar or a dinner party.) In a calm, almost jocular manner, tell the boy-next-door that you admire his taste in men, that you are aware of his designs on your man and that you do not appreciate them and that you wish he would find a boyfriend of his own. Offer to help him find one. He will flatly deny trying to steal your man. Your response should be "I'm glad to hear that because I would like to be your friend but this is in the way." Any one else who hears this exchange will immediately excuse himself and text this juicy gossip from Battery Park clear up to Washington Heights. Once all the cards are on the table, the boy-next-door will probably fixate on someone else. This is called defusing an infatuation.
By now, I'm quite sure that all our readers are jumping up and down with impatience because I have not gotten around to the crux of the matter: your unhealthy fears and your need to control what is beyond your control. Take stock of your feelings. Share them with your partner, friends and maybe a professional. You need to shed your hostility.
You should realize also that in a world in which gay partnerships are fluid and ill-defined, and, in a world in which half of all marriages end in divorce, almost anyone is fair game for the batting of eyelashes. Also, not every man on your block is a gentleman with a code of behavior to match your own. This will never change. Let go of it. A world in which no one hits on your partner is a fantasy, and your partner's behavior in this situation is the only one that matters. His refusal to "do anything" about the boy-next-door may not be particularly valiant, but if he says you have nothing to worry about, you should trust him. Meanwhile, if the building next door should come up for sale....