The Master had been in the Doctor's 'care' (also known as 'captivity') for three weeks and the Doctor still hadn't shagged him. That was exactly the sort of goody-goody self-depriving thing the Doctor would do. Possibly he was doing it as part of the Master's punishment for having tried to take over the universe. In any case, the Master thought it was time they got with the hot alien sex.
"We have centuries of sexual tension to work out," he said, quite certain he was being reasonable about it.
"That's not sexual tension, that's mutual loathing," said the Doctor, not looking up from rewiring the innards of his senile old TARDIS.
"Same thing," said the Master. "Don't think I've forgotten the many torrid nights we spent together on Gallifrey. And the mornings. And the afternoons."
"You weren't evil then," said the Doctor, who probably thought this was a winning argument, but he was wrong.
"Don't tell me you're not attracted to evil. All good people are. You're the most virtuous being in the universe, which means you're given to spontaneous orgasms in the presence of someone like myself. And that wire doesn't go there."
The Doctor inspected his work with an obviously faked casualness. "You know nothing about my orgasms."
"I think you've developed a fetish for not getting any. How else could you have travelled with Martha Jones all that time without giving her one over the console? Not to mention how frigid you must have been to avoid Frisky Jack's libido."
"I'll have sex with you if you stop being evil," said the Doctor, finally looking up from his work.
"I'm not evil, I'm just differently moral. You shouldn't be using such outdated and frankly offensive terms." The verbal sparring was almost as good as the sex. Well, it wasn't, but the Master thought he might as well enjoy what he could get.
"Besides, you're technically my prisoner."
"And by 'technically' you mean 'actually'."
"So it'd be taking advantage." The Doctor looked quite satisfied with this line of reasoning. The smug git.
"I don't mind."
"I mind. I'm not betraying my moral centre for a quick one up against a wall."
"Think of it as a marriage. What's a relationship but two people trapped together due to circumstance and shagging occasionally to combat the sheer tedium of such an arrangement?"
"I have an STD," said the Doctor, clearly having run out of actual arguments.
"Kinky. I can cure that with my mighty penis."
"I'm not falling for that one again."
The Master was losing patience with this silliness. "I'm not asking you to let me take over a galaxy, I'm just demanding some sex to alleviate boredom. And," he added, appealing to ego, "you're quite a good shag. Or at least you used to be. Possibly by now your cock has shrunk into your body and your balls have fallen off." He'd tried to be flattering, but he just couldn't help himself. That was part of the downside of being evil.
"Okay, fine."
"What?"
"Let's do it. Let's have sex. We can use your bed, because I'm not getting my nice new sheets all messy."
The Master was somewhat confused by this conversational turn. "Are you serious?"
"Might as well. It's not like I can take you out for dinner and a film. That and I'm pretty much gagging for it."
"For some reason I've gone right off the idea of sex." He hadn't really, but this was too good an opportunity to miss. Again, the downside of being evil. Stopping people getting things they wanted was often a reward in itself.
"You're lying. I can tell when you lie."
"In fact, I think I'll go and curl up with a book and a mug of cocoa. Gosh, what a tiring day it's been."
"You haven't done anything! You sat in the cloister room all day setting fire to my paperbacks!"
"Minor vandalism takes such a toll on the body. I'll see you in the morning, after you've had your morning wank."
"...git."