The protagonist and the romantic interest go to bed with meins so somber that one might think they were going to the gallows. Writers could do worse than to put the play back into foreplay. A giggle does no harm to the hard-on that knows what it is about. Spontaneity in general is not compatible with good writing, but the appearance of spontaneity is. The erotic scene must seem to be, at its climax, both spontaneous and inevitable.
Sincerity covers a multitude of sins.
A new writer is well advised to stick close to scenes that he himself finds irresistibly arousing. Even if that scene is a bit out of the ordinary or not to a particular reader's taste, the writer's excitement will communicate itself as surely as a college boy gets horny when his roommate jerks off. Some writers admit to masturbating while they write their erotic scenes, but that is not so easily done by the touch typist. However, no erotic scene was ever harmed by being drafted when its author was exceptionally horny. One writer I know of, to keep himself honest and to maintain a keen edge on the erotic scene, refused to write fictional erotic scenes until his career was fairly advanced.