From Odysseus to Brad Pitt, heroes are often measured in part by their sexual conquests. Why do you think there is such a derth of sexual bragging or actual sex in Beowulf?
I have absolutely no clue as to the correct answer to this one, but in pondering the previous question I answered regarding gold/light/purity, I wondered if perhaps the Christian influence had something to do with the lack of sex.
When the Christian monk(s) wrote this down, isn't it possible that they might have wanted to do away with the "evils" of sex? They were glorifying brave men who fought for noble reasons, not Viking berserkers who raped and pillaged. It seems to me that if there ever were sexual escapades in "Beowulf," those would have possibly been "edited for content" by well-meaning Christians who wanted to keep the evils of sex and women out of this story.
The only mention of sex I found in my reading occurred in lines 662-665:
"Hrothgar departed then with his house-guard.
The lord of the Shieldings, their shelter in war,
left the mead-hall to lie with Wealhtheow,
his queen and bedmate."
And this wasn't some wild affair between a single warrior and a saucy wench - this is an old married couple, Hrothgar and Wealhtheow, and it could even be that all they did was go to sleep!
On the other hand, perhaps the original scops of the story of "Beowulf" didn't include any conquests for the warriors. Perhaps it was more important for them to talk about the glory of warfare than it was to worry about those unimportant women-folk! :)