This is an excellent question.
There are many different forms of ritual impurity (
tumah). The
seven clean days need to be clean of several forms of ritual impurity – not only
niddah, but also
expulsion of semen.
However, the ritual impurity of discharging semen is not on the level of
niddah and does not affect the halachic permissibility of marital relations, (which themselves entail transfer of semen).
Please be in touch with any further questions.