Maurice is, at its heart, a classic doomed love story: Maurice meets Clive; they fall in love; they pursue a secret relationship because Society doesn't approve; Clive breaks Maurice's heart by eventually marrying someone acceptable so that he can have a career in politics.
Clive never gets over Maurice, really. But Maurice can't hang around forever being Clive's Secret-on-the-Side. In many books (and modern movies), these two would buck society and reunite, living happily ever after. I can just hear the dramatic music that would play during the Hollywood Ending! Would Justin Timberlake be too old to play Maurice?
But EM Forster was a realist. He knew that Clive would never stay "a confirmed bachelor", or even keep Maurice "on the side". His political career was way too important for that. Plus, Clive did a pretty decent job of lying to himself.
Instead, Maurice elopes with the gamekeeper. Personally, while I love a happy ending -- and I love this happy ending -- I feel that Forster kinda tacked it on at the end to make a point.
And that's the thing with Maurice: aside from it being a really sad doomed love story, it's also a discussion on how being gay shouldn't be a crime. Maurice spends a good deal of the novel trying to become "normal", but he can't do it. He can't undo his own nature. The whole point of showing Maurice doing this is to make him sympathetic and show that even though he doesn't want to be this way (it's not a lifestyle choice), he can't help it. He's born this way, baby!
Now, as for the gamekeeper, Alec: I would have liked it to have taken more than 10 pages for Maurice and he to go from "who the fuck are you?" to "you wanna come fuck my brains out?", but given that Lucy and George in A Room With A View go from sharing Meaningful Glances to macking in a field also within about 10 pages, all is forgiven. Plus, George and Lucy have to elope to get married and who knows what happens after with them, either? Just like Lucy and George, Maurice and Alec will end up living at the edges of society, not really ever fitting in.