There are any number of reasons why this is a bad idea. John has a wife, children. The tabloids would crucify them. He'd have to quit his dream job, because the Doctor having an affair with a married man wouldn't sit at all well with the BBC. It even goes against his own moral sense, passed down very clearly by his parents, bless their hearts and rest his mother's soul. On paper, hell, by any sensible measurement, being the 'other man' is incredibly, incredibly foolish.
But the heart is, he has learned, so very foolish and rarely sensible.
There's so much work. He packs his day full of it, or has it packed for him by the relentless filming schedule. When there's a break, he squeezes in a few one-offs and radio dramas, just to keep his hand in. He loves being the Doctor but it's overwhelming at times, the sheer scale of it. It's not just a job, not remotely. It's interviews and tabloids and ceremonies and everyone everywhere he goes knowing his face, his name, and wanting his time. He's grateful that he knew what he was getting into, at least in part. He's grateful that he acts because he loves acting, not because of fame. He doesn't consider himself famous, even though he knows he is. He's grounded, and is surrounded by people who are grounded too, because it is ultimately a job and not his life, no matter how institutionalised it makes him.
When he has any free time at all, it's spent at home, collapsed in exhaustion, barely leaving the bed or the sofa. He doesn't have time to socialize during filming, much less the energy, so the only people he meets are his co-workers. He's so many friends on the crew, in the cast, the production team, the writers. They're each other's support network, all the Cardiff teams, trying to work around each other as they film their respective series. It's a whole community of people, pouring out everything they've got for one mutual goal: to get the episode in the can. To make something that will dazzle the crowds.
It's not very surprising that there've been so many romances on set. Marriages, even. It's also not very surprising that he's bonded so much with his co-stars. Billie was the first, platonic but very deep. She was the one who he turned to when he was the new boy, and for that year they really were inseparable. John -- Barrowman, that is -- was another one, even though they weren't on screen together until ages later. And then there was Sophia; with her he was anything but platonic. If she hadn't buggered off to California they might still be together. But then he might not have found John, his John, and he can't imagine that.
There's no future in it, not in the traditional sense. There's no kids, and he still wants them. There's no wedding, no public announcement, they can't tell their friends. They can't live together. They can't see each other very often, even, though when they do it's incredibly intense for all the waiting. To the world, all they can ever be is friends, and that has to be enough, because he's not the sort of man who destroys families, reputations, careers. He isn't. He wasn't raised that way.
But he loves John. He does, with all his heart. With both hearts, when he's the Doctor. He's only flesh and blood.
Strangely, he thinks he better understands his ancestors now. He was so shocked by them, William and James, shocked at the violence they were at the centre of, at the signatures on the Covenant. He didn't understand how they could do something so terrible out of apparent love for their families, their country. He is at least under no illusions that this is the righteous choice, this affair.
He tries not to think about the morality of it. He tells himself they can keep this to themselves, keep it private. As long as nobody finds out, nobody will be hurt, and friendship is an excellent cover as long as they're careful outside of closed doors. It's useful that he's adept at separating his private life from his public one.
He's fairly certain that John loves him. They're not much for declarations, but he sees so much in the small gestures, the fond looks, the heat in his eyes. Whatever the world might say, there's something pure when they're together. Something wonderful.
When filming ends, before he's pulled away to the RSC, he's going north with John for a week in a secluded rental. It's going to be glorious, the longest time they've had together since the Master died. He's suggested to Russell that the Master should return before he ends his term as the Doctor, which isn't yet but will come as all things do. That's one thing the tabloids are right about.
There's always an ending, to every job, to every relationship. He understands that, just as the Doctor does. But he'll hold on as long as he can to the things he loves. That's the sort of man he is.