Remarkably perhaps, but
details of the ancient Britons' sex lives have survived the ages and there
is an account which suggest that some kind of wife swapping took place among
the ancient British.
This is a typical example (1).
"The population is exceedingly large, the ground thickly studded with homesteads...and the cattle very numerous...hares, fowl, and geese they think it unlawful to eat, but rear them for pleasure and amusement...Most of the tribes do not grow corn but live on milk and meat, and wear skins. All the Britons dye their bodies with woad, which produces a blue colour, and this gives them a more terrifying appearance in battle. They wear their hair long, and shave the whole of their bodies except the head and the upper lip. Wives are shared between groups of ten or twelve men."
This is Julius Caesar’s account. It is important to realise however that this is only one translation and Roman Latin is remarkably vague and is often possible to make more than one interpretation and as I am no Latin scholar I have no way of knowing if it is correct one. There do not seem to be any other sources for Caesar’s claim.
This is another and fuller translation from a much more authoritative and excellent source even if it is not quite so clear(2)
Ten and even twelve have
wives common to them, and particularly brothers among
brothers, and parents among their children; but if there be any issue by
these wives, they are reputed to be the children of those by whom
respectively each was first espoused when a virgin.
Perhaps it was a misunderstanding on Caesar’s part, it’s possible that some of the larger roundhouses were divided up into segments some modern reconstructions have separate rooms at the edge of the space underneath the eves and these could easily have been private bedrooms – apartments almost, so perhaps many families/couples could live under one roof (the poor?) and this gave rise to the confusion.
I have to say though, much as I like the idea of multi-roomed roundhouses, (and why just one storey?) it seems far more likely that Caesar is describing the situation in Ireland where matriarchal societies were suppose to be still flourished at this time.
I hope to update this
soon but there are good arguments to suggest that Ireland was very different
culturally to mainland Britain, probably far more back then than now, and
that Caesar at that time had very little knowledge of the British Isles, he
would only have known of south east England and probably assumed that everything
beyond there was very different and backward but this wasn't necessarily the
case. I have found out quite a lot about the marriage customs, etc of the
early (mainland) British and it wasn't very different from now - a bit boring
(1) not sure about this site but you get the idea
(2) this is from the wonderful Gutenberg Project where many ancient sources can be found online
book V, X1V