An interesting aspect of the Mythological Cycle is that we can catch a fairly good glimpse of how the ancient Celts felt about sexual relationships, judging by the way the actions of their gods were viewed. It would seem overall that issues like chastity and fidelity are not the issue per se, but rather the concept of rightful ownership as it relates to spouses.

Consider, for example, The Taking of the Fairy-Mound, in which the Dagda craves the company and favors of Boann, the river-goddess:

Apparently, adultery carries no real moral stigma; Dagda, chief of the gods, commits no sin in desiring another's wife, and he holds nothing against her for returning the sentiment. But unlike Zeus, who would have just had his way with her without regard for consequence (or whether or not she was willing!) the Dagda shows a very gentlemanly side:

The problem is not that these two wish to have an illicit affair; the problem is that her husband's feelings will be hurt, and his pride stung because something that belongs to him acted without his knowledge or consent. Apparently there is no harm done so long as Elcmar doesn't find out. Of course, Boann conceives and brings forth Oenghus, who bears no stigma for being illegitimate, but is rather the darling of the Tuatha.

A second example can be found in The Wooing of Etain, who after wedding king Eochaid, found that her brother-in-law, Ailill, was wasting away from some strange sickness. Like a good sister, she tends him daily.

Ailill then confesses that his wasting comes from desiring Etain. Wrestling his passion for more than a year was killing him.

Etain devises a plan for them to meet at dawn just outside the stronghold's walls, where she would give him her "embraces." And not for a moment does anybody suspect her of being a bad wife; she swore to Eochaid that she would do everything she could to cure his brother. If that means having sex with him, then so be it. The tryst never happens- the whole thing was an elaborate scheme by Midir, Etain's first husband from another life, to get her back. Midir asks Etain if she would return with him to his sidhe, and Etain answers that she would do so only with Eochaid's permission. Midir leaves her and the story ends well, with Ailill relieved of his passion and Etain's honor intact, and the king unshamed.

Although it can seem confusing, Etain behaved in a way that best agreed with the ancient Irish sense of honor; a wife is to be loyal to her husband. The conflict arose when her sexual fidelity had to be compromised in order to fulfil an oath made to her husband. The conflict in that was not a moral paradox, but a matter of geography. By leaving the fortress and taking care of business out in the woods, Etain would have done no harm, as she hadn't shamed the household of Eochaid. She proves that she is no wanton when Midir asks her to come with him, because despite everything she has done, she remains faithful to her man.