Remembered another one; sex scene between Jude Law and Rachel Weisz in Enemy at the Gates. They're both Russian soldiers fighting in the seige of Stalingrad, and they do it in the makeshift barracks among all the other troops, watching them trying to stay quiet is both tense and hot.
We're not talking the sight of Robin Askwith's hairy arse, here. There's a scene where he does a job for a posh, middle-age woman who abhors dirt. Naturally, the clumsy sod gets mucky, and she goes about trying to strip said "filthy boy" and intends to have her way with him.
More than a twitch of trouser-arousal at that one, but maybe that's just my interest in older women/domination/infantilism, etc.
In the words of Askwith himself: "Phwoar!!!"