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Sexual relations when a woman is a nidda – from the time when her period begins until she goes to the mikveh after it is over – is forbidden. Transgressing that negative commandment carries with it the punishment of karet – the people will be “cut off” from the community. The Mishna (14b) discusses the case of someone who transgresses a positive commandment of nidda and explains that the positive commandment is when a couple is engaged in permissible intercourse, and the woman suddenly realizes that she has become a nidda. When she informs the man of her status, his obligation is to remain in place until his erection is lost in order to avoid sexual pleasure with a nidda.
This parallels the case of the individual who enters the Temple and becomes tameh while he is there, since in both cases the entry was permissible, and the problem developed at a later time. The difference is that in the Temple, the man who became ritually defiled must leave as quickly as possible, while in the case of the nidda the recommendation is to wait until it is appropriate to leave.