Is it not simply because most characters are male? I seem to remember inventing some awful soap-storyline for the Lord of the Rings characters which, of course, had to involve copious amounts of amorous scenarios. Men together, yes; but what about the men and the women (disgraceful stuff involving Arwen), and lots to do with Hobbits.
It's not all to do with sex but there are only so many basic storylines.
People want to make fun of stuff. It's surely a sign of health if one can take everything with a pinch of salt and, yes, laugh at everything. When playing Lord of the Rings risk, a certain other Barrowdowner and I make constant ridiculous jokes—some crude, but more often than not detailing farce or the absurd.
The bureaucracy of leaving a single Orc to defend the Shire in times of war.
It just so happens that sex is as simple, as crude, and as offensive as it gets; and (sometimes) very funny.
Watch any film, any television show, any cartoon, and be aware that at the same time, somewhere in the world, a group of teenagers are watching the same thing and laughing about how gay it is. We can take this issue for the Lord of the Rings if we want, but I think it's a bit of a waste of time.