We appreciate the sensitive nature of this question.

Yes, even when taharat hamishpachah is observed, it is prohibited for a Jewish man and woman to have sexual relations outside of the context of marriage.

According to the Rambam, relations without halachic betrothal, kiddushin, fall under the Torah level prohibition of "lo tihiyeh kedesha mibnot Yisrael", (Devarim 23:18; Mishneh Torah, Hilchot Ishut 1:4). This is independent of whether the woman is a virgin, in niddah, or unmarried, and whether or not the man is married.

Even being alone together is a violation of the prohibition of yichud. (See Talmud Avodah Zarah 36b.)

It also bears noting that unmarried women are prohibited by rabbinic edict from immersing in the mikveh.

Please write back with any further questions.