The perception that the Celts were promiscuous seems to be based on, at least in part, ancient writers’ interpretations of marital relationships and / or a superficial knowledge of Celtic customs and culture.
On the latter point, Strabo admits to lacking evidence according to David Rankin in Celts and the Classical World,
... Strabo who says that Britons not only cohabit with the wives of
others, but with their own sisters and mothers: he admits, however,
that he has no reliable evidence for these assertions.
On the former,
Both Caesar and Dio Cassius describe types of communal marriage or
polyandry, with kinsmen sharing wives and the descent becoming in
effect matrilinear.... it is perfectly possible that multiple marriage
was common among the Celts, at least in Gaul.
Source: John King, Kingdoms of the Celts (2000)
Rankin is goes into more details on Celtic marriages:
Certainly we know that both Irish and Welsh systems of marriage
recognised various marital categories. Also categorised were unions
not of marital status, but which also were taken into account from the
point of view of compensatory payments, as were the more permanent
bonds. It would be easy for foreign observers to remain unaware of the
various ramifications of a system which recognised, say, eight or nine
categories of union, and in the case of Old Irish Law, three classes
of legitimate wife.
This must have seemed very alien to Roman observers (assuming they even understood it), especially considering
The city states of Greece and Rome had highly organised political
structures which allowed no place for women in power. Greeks and
Romans were all the more astonished at the relative freedom and
individuality of Celtic women.
Rankin mentions the Greek city states here but he fails to note that Spartan women had more rights than those in other Greek city states, and that Spartan women could sleep with another man for the purpose of procreation if the husband agreed. As a consequence, Spartan women were regarded as being promiscuous by other Greeks, a point which actually lends weight to Rankin’s argument.
We should also consider, as John King does, that
Dio Cassius no doubt intended his account to be shocking to the Roman
sensibility, and the accounts of polyandry have subsequently been
attacked as no more than propaganda to discredit the Celts as
Further, this kind of propaganda was used to combat potential ‘lapses in Roman morality’ as shown in the case of a queen of the Brigantes in northern England (cited by King)
Queen Cartimandua's elopement and alleged sexual
promiscuity....scandalized Roman society, and was long cited as an
exemplar to Roman matrons of the misery awaiting them if they
succumbed to barbarian patterns of lasciviousness.
The issue of promiscuity inside and outside of marriage does not seem to have been considered by Roman writers, and one of these writers – Caesar – broke his marriage vows quite a few times. One wonders also if they considered the large number of brothels, prostitutes (male and female) and mistresses that Romans at all social levels indulged in.
It seems fitting to conclude with Rankin’s observation on the whole matter of sexual relationships:
Of any tribe’s customs, those most liable to misunderstanding by alien
observers are those which concern sex.
Paul Cartledge, 'The Spartans'
D. M. MacDowell, 'Spartan Law'
Ray Laurence, 'Roman Passions'