The gemara says the inside of the straps of the head tefillin should not be red on account of 1) disgrace and 2) ANOTHER THING. Rashi says the second item refers to the fear that people might say the wearer of the tefillin cohabited with his wife WHILE SHE WAS A NIDDAH. This implies that he could cohabit with his wife while wearing tefillin if she wasn't a niddah. If one would not be allowed to have relations while wearing tefillin it seems that the Gemara would not bother to worry about forbidding red for a remote possibility. What is your understanding of Rashi?
Barry Epstein, Dallas, USA
It is true that the possibility seems remote; however, many times there are Takanos, enactments, in the Gemara where one reason is given, and there are other reasons which the Chachamim chose not to reveal.
As for your specific question, while it is prohibited to engage in such activity while wearing Tefilin even when one's wife is not a Nidah, it would seem that one who was not careful about the severe prohibition of Nidah would not be careful about the prohibition of not cohabiting with Tefillin.
D. Zupnik