Get your mind out of the gutter.
It is simply not there.
It's a children's book.
And JKR said in an interview that it's nothing.

http://www.leakylounge.com/lofiversi...hp/t34788.html (half-way down the page)

Q: What on earth was Aberforth Dumbledore doing with those goats?
[Big laughs from all]
JKR: Your guess is as good as mine! [Evil laugh!]