If there are multiple characters to eventually bone, there is absolutely nothing wrong with boning a character early in a story. A good example of this is Working Man. You bone a number of characters over the course of that story, and it has a hot ending that ties it all together quite nicely - that man is one lucky guy!
Of course, the developer of Working Man wasn't milking the story for all it's worth. He just made a nice game and was done with it.
While some people might lose interest after boning D, I for one would want the next step. I.E. a foursome with Graham, D and Olivia, a threesome with Elena and D. Oh and the opportunity to bang Koko possibly in a 3 way with D...