Among the biblical prohibitions that would lead to a death sentence according to Torah law are cases of incest. The Mishna on today’s daf lists those situations – like sexual relations with one’s mother, one’s father’s wife or one’s daughter-in-law – that lead to the punishment of sekilah – of death by stoning.
The Gemara quotes a baraita where Rabbi Yehuda teaches that in the case of incestuous relations with one’s father’s wife, for example, if the marriage was “not suitable,” and it was forbidden for the couple to have married, then the relationship is not considered incestuous.
What does the baraita mean when it says that the marriage was “not suitable”?
The halakha recognizes two different types of forbidden marriages. If the prohibition was on a level that the punishment would have been severe – i.e. the punishment would have been karet (being “cut off” – a reference to divine retribution) or mitat beit din (death penalty carried out by the court) – then no marriage can be said to have taken place at all. Any marriage ceremony that is performed, for example, on behalf of a brother and sister, has no meaning whatsoever; he is not her husband and she is not a married woman. A second type is when the prohibition was merely a lav – a simple prohibition. In such cases, e.g. when a kohen marries a divorcee, according to most opinions the marriage does take effect, even though it is incumbent upon the Jewish court to try to force the husband to divorce his wife, since they cannot sanction the couple living together.
The Gemara suggests that Rabbi Yehuda may agree with Rabbi Akiva who rules that even in the latter case where the prohibition is a mere lav, the marriage is forbidden and has no significance in Jewish law. Were that to be the case, even if the prohibition were a simple lav the marriage would be seen as not have taken effect, and the cases of incest described in the Mishna would not be significant.