I understand the practical difficulties associated with a Labor-Lib Dem coalition (among other things, it would rely on the votes of smaller parties). What I don't get this is objection that such a coalition would "have no moral legitimacy." It's true that the Conservatives won the most votes of any party, and that the Labor Party lost a lot of seats. It's also true, though, that left-of-center parties won more votes than right-of-center parties. The electorate displayed no clear majority preference, but the preference for left-of center government over right-of-center government seems as clear as any other.
I don't see any reason why one principle of forming an electorate should hold sway over any other. If the Conservatives and the Lib Dems can agree on some governing agenda, then more power to them, and the same holds for Labor and Lib Dems. Whatever majority coalition of parties manages to come to terms seems, by definition, to be the correct one.