Thank you for reaching out to us on this important matter.

It is prohibited for a Jewish man and woman to have sexual relations outside of the context of marriage. This applies equally to people who have always been single and to those who are divorced or widowed.

First of all, the prohibition of seclusion, yichud, applies to any man and woman who are not close family members or married to each other, (Talmud Avodah Zarah 36b, Shulhan Aruch Even haEzer 22:2).

Additionally, according to Rambam, a couple are prohibited on a Torah level to have relations outside of kiddushin (halachic betrothal), as an offshoot of the prohibition of cult prostitution, (Devarim 23:18, Rambam Ishut 1:4).

Furthermore, unmarried women are not permitted to immerse in a mikveh and therefore generally have the status of niddah, (Shu"t Rivash 421, Bet Yosef Yoreh Deah 183). A couple are prohibited on a Torah level from having relations when the woman is in niddah (VaYikra 18:19).

When a couple are prohibited from relations, there is an additional prohibition of having intimate contact (derech chibah) with each other. According to the Sifra and then the Rambam, this is a Torah level prohibition that applies even to hugging and kissing that may lead to relations, (Sifra Aharei Mot 9, Sefer haMitzvot 353, Mishneh Torah Isurei Biah 21:1, Shakh Yoreh Deah 157:10). According to Ramban, this is prohibited on a rabbinic level, (Hasagot HaRamban to Sefer HaMitzvot ad loc.).

There is debate as to whether less intimate physical contact is prohibited on a Torah level, (Orhot Hayyim Hilchot Biot Asurot 13), on a rabbinic level (Bet Shemuel Even HaEzer 20:1), or as a matter of custom.

Please write back with any further questions.