To put it simply, punishments for the sin of fornication in the medieval world depended upon the marital status of both parties. Premarital intercourse between two young people who were both unmarried but intended to one another carried less of a penalty than adultery involving one or both parties. For Ivo of Chartres fornication was a sin that required confession, absolution and penance (often several years removal from the sacraments), and an earthly punishment such as a fine and/or a corporeal component such as whipping. Whipping was common in England for example, and would occur publicly in full view of the population for instructive purposes. It could also happen close to the parish church or stretch through the streets of the village to compound the shame of the offence.

While it is true that in cases of adultery it was not uncommon for men to murder their spouses and occasionally received a degree of licence to do so from some authorities we should remember that this fundamentally a different kind of thing relative to simple premarital intercourse. Moreover support for such violence was hotly contested and Church authorities often argued strenuously against this kind of vigilante behaviour. Brundage has written at length on this and contends that scholars such as Gratian in the twelfth century argued vociferously that adultery was a matter for public authorities and not private vengeance. Offenders should be forced to do public penance for their crime, receive a punishment and eventually rejoin the community.

A few things to consider which might complicate this question

In the twelfth century there was an unfolding argument regarding the nature of marriage and when exactly a marriage was constituted. Scholars such as Gratian contended that marriage was not merely about the consent of the betrothed it was also about the union of the physical bodies of the intended. Marriage in this context was only completed or perfected when sexual consummation had occurred.

For fellows such as Hugh of St. Victor or Peter Lombard this was not the case. For them it was consent and consent alone that made marriage. To insist upon intercourse as a central component of validating a marriage was to devalue the relationship of Mary to Joseph in the Holy Family and therefore to call into question the entire nature of their relationship.

With these things considered one can imagine how complicated the question of marriage would be to contemporaries, particularly those far removed from the intellectual academies of the Latin Christian West who had no exposure to these lofty theological arguments.

Thus was a man an unlawful fornicator when he had intercourse with his intended? If they had exchanged promises were they indeed already married? At what point did they pass over into the realm of man and wife when it was very common for individuals to have private celebrations or none at all? Quite often there would not even be witnesses to their union, and it was by no means a given that a priest would ever be involved in the process. As Brundage notes it was an entirely valid defence to claim that one earnestly believed that the woman they were having sex with was their true spouse and wife. This emphasizes the degree of uncertainty at the heart of many of these cases. James Brundage, Law, Sex, and Christian Society in Medieval Europe (Chicago: University of Chicago Press, 1987), 247.

It is also important to remember that children conceived before a marriage had taken place could be retrospectively legitimated if the parents of the infant became married subsequently. There was therefore an ideal standard that people seem to have been aware of but authorities did recognize that people were people and often fell short. So long as they "did the right thing" thereafter such incidents could often be resolved.

The immediate family members of either party might intervene as well if they discovered the relationship had crossed over into copulation in order to ensure the wellbeing of children and the honour of the woman involved.

Shannon McSheffrey has noted that it was very common for men to "dangle marriage" in order to entice a potential partner into intercourse and so women were advised to be wary of aspiring partners and their intentions.

McSheffrey also encourages us to consider the similarly for contemporaries between concubinage and premarital relations occurring among people who were loosely intended. Shannon McSheffrey, Marriage, Sex, and Civic Culture in Late Medieval London (Pennsylvania: University of Pennsylvania Press, 2006), 69.

As for instances of pre-marital or extra-marital intercourse in the context of sex work the question is rather complicated. Men such as Thomas Aquinas readily admitted that prostitution for example was an evil, but nonetheless recognized that it was a necessary one. Leah Lydia Otis notes that for him sex workers were a means of ensuring that sexual appetites did not boil over and cause deeper social problems.

Aquinas famously compared prostitution to a sewer for lust so that male sexual passions did not accumulate and lead to the endangering of good or virtuous women. It was not uncommon for clergymen to be land lords for brothels in large cities and receive rents drawn from their revenues. Leah Lydia Otis, Prostitution in Medieval Society: The History of an Urban Institution in Languedoc (Chicago: University of Chicago Press, 1985).