Flynn knows it's necessary. He knows he'll be too tight if Yuri doesn't open him up first; it will hurt. He just doesn't like it. Yuri's fingers tease in and out of him, and any pleasure he feels leave him as the other man's fingers do. It's nothing compared to his cock, and the smallness of his fingers just leaves Flynn wanting it even more.
It's always a tight fit, at least at first. Flynn is always surprised at how that first finger seems to stretch him a bit, and he's even more surprised at how it always manages to hurt too. But Yuri is gentle, and he opens Flynn with the ease of experience.
When the second finger enters Flynn doesn't feel it as much as the first. It's a bit of a pinch, but that's all. Yuri, seeming to sense his impatience, slowly inserts two more, and Flynn feels at first as if he's being pulled apart. As Yuri tugs him farther open, the pain spikes and then decreases in a flash. All he can feel is the sensation of Yuri's fingers inside of him. Just when he thinks Yuri is done, there are the fingers again, all around his ass, and he rolls his shoulders at the feeling.
When Yuri's fingers are finally gone, the emptiness is a cold shock. It's almost a relief, even though it still hurts somewhat, when Yuri's cock finally enters him.
If Yuri takes too long, Flynn eventually snaps at him to hurry up, which, of course, often has the opposite effect. Yuri takes his time, and he isn't going to be rushed, especially in something like this.
Flynn knows he'll be grateful in the morning when his ass is less sore than it would be without preparation, but it still takes an annoyingly long time.