Celtic Marriage

The Celts had an interesting attitude to marriage. According to the Celts, there was no marriage ceremony to formalize things. To be considered married, all you had to do was lay together with your beloved. Usually with the intention of being together. This was considered to be legitimate. Seldom was there a marriage feast. Sometimes not. Still, everyone considered it to be important to be married.
Many children were illegitimate. But there was no stigma attached to the illegitimacy. Some people from myth were illegitimate. Like Connor, the king of Ulster. He was the son of Cathbad the druid and not his mother's husband. Cuchulain of Ireland myth was said to have parentage from Lugh the Longhand, and Salthuf, his human father. Sometimes, illegitimate children would not have it easy.
Infidelity was considered to be humorous to the Celts. It wasn't considered to be shameful for a woman or a man to dally with members of the opposite sex. Even among the Tuatha de Dannan, infidelity wasn't considered to be at all shameful. Still, fidelity was considered to be the norm among the Celts.
As for Rape, it was considered to be a crime. Although the victim wasn't shamed, as the crime was thought to be entirely the fault of the rapist.  Also, the rapist didn't gain the woman, unlike other cultures.