When Mycroft says no, it marks the end. End of argument, end of discussion, end of don't you even think about mentioning it again. He can dish out no but he can't take it, and Greg is aware of this because he's tried it.
The first time a black car arrives at one of his crime scenes and he receives a text telling him to get in, Greg takes one look at the phone, at the car, and texts back Piss off. Because he doesn't take orders from people who won't even show their faces.
In retrospect he shouldn't have been surprised when, an hour later, the car comes back equipped with two men twice his size who wrestle him into the back seat. And when he meets Mycroft Holmes, it's with his clothes rumpled (moreso than usual) and a bruise blossoming along his jaw.
Where Sherlock is a tornado, tearing through anything and everything in his path not smart enough to get the hell out of the way, the elder Holmes is more of the hurricane variety. There is no running, and there is no hiding, because Mycroft Holmes does not take no for an answer.
Apparently, he doesn't respond well to Piss off, either.
Greg doesn't tell Mycroft no again. Not until a year later and three drinks too many and Mycroft's mouth is on his. They've danced around this for months. Perched on this precipice and teetering between too far and not far enough. Now that it's finally happened, now that Mycroft has tipped them right into that too far realm, Greg isn't complaining. Sure as hell isn't minding except—"No," he breathes, and it takes more resolve than he'll admit to put distance between them while Mycroft's expression is darkening, then smoothing out into quiet apathy.
"I'm married, Mycroft." As though that means anything to him anymore.
Mycroft smiles one of those patient, cool smiles that Greg hates because it's such a far cry from the warm, soft curve of his mouth he wears when he thinks no one is looking.
"Do you suppose," Mycroft asks, "your wife has given such protests to the lovers she brings to bed?"
A year ago, Greg would've decked anyone who said such a thing, British government or not. Now the words only dig at a wound that is still fresh and it makes him withdraw and shut down completely, the mood spoiled, any chance of Mycroft convincing him to change his mind out the window.
Mycroft makes it worse by asking, "What are you afraid of, Gregory?"
He leaves, Mycroft's expression doesn't change, and Greg wants to take that condescending face and tear it to pieces and make Mycroft feel as vulnerable and raw and lost as he does.
Mycroft bloody Holmes, the ice man. There have been moments in time, brief glimpses, wherein Greg has thought that name was far too harsh. The Mycroft he knows can be pliant and warm...so long as he's getting what he wants, and utterly immoveable and frigid when he doesn't.
Two weeks later, Greg comes home to his wife packing her things and he isn't surprised. She leaves for her sister's, and Greg leaves for the pub. And somehow, somehow, Mycroft knows this, too—go fucking figure—because there's a black car outside his house when he returns. Inside his flat, Mycroft is seated on the couch, in the dark, back straight, hands folded atop his umbrella handle, and Greg doesn't know what to think. Partly because he's half-drunk, but mostly because Mycroft's never come to see him in his home. Hell, Mycroft doesn't come to see him period.
He braces himself for the hurricane, for an I told you so. Something. Anything he doesn't want to hear.
Mycroft rises to his feet, looking horribly out of place in the sparsely decorated living room like a diamond in a pile of ash.
The next thing is remarkable in Greg's eyes; this icy, immoveable man moves toward him, umbrella hanging off his arm, and he takes Greg's hand warmly between both of his own and says, gently, "I'm so very sorry, Gregory."
Greg's mouth twists, because he knows Mycroft isn't sorry at all. Not really. But he's standing here trying, even if he doesn't mean the words, because there's a heart in there after all and it's a heart that doesn't like seeing someone dear to him hurting.
It isn't difficult to come up with an easy retort of, "Piss off."
Mycroft's mouth curves into a warm smile.
Greg kisses him first this time, because the immovable Mycroft Holmes budged, even if only a little, just for him.