However, in the past, it was highly frowned upon. When I say frowned upon, I also include that fornicators in the law of Moses were stoned to death. Muslims still have strict rulings in regard to Chasity.
In traditional patriarchal societies (eg Mosaic, ancient Roman, many others) this is something enforced against daughters and against married women by their own birth-families and then by their husbands, possibly with state sanction. It was not something generally applied against widows, or against men. Although a man who had sex with a married woman might sometimes be killed, this was less common - the woman's family might not wish to get into a vendetta with the family of the dead man. Also most societies had a significant class of prostitutes and other unattached women who had left their families and against whom the law was not applied, because it was primarily family law, not state law. If the state intervened at all it would be to prevent, punish, or sanction the actions of the family.So you could have a fairly traditional society in your fantasy campaign setting, while still having sexually independent female characters, detached from their families. Although they may be regarded as 'loose women', it's unlikely that anyone will try to kill them for fornication.