Marriage, as instituted by Yahweh in Eden, and defined by the Hebrew Scriptures, consists of a commitment to faith and physical fidelity between only one man and one woman for their entire life, for the purpose of producing “undefiled seed” or “righteous offspring” (Mal 2:15, Gen 9:1, Psa 127:3 et al).
Betrothal describes a preliminary "hands off relationship" between the two who are committed to marriage, having sought parental guidance or other appropriate counsel, and unless widowed have retained their virginity (Lev 21:7,13; Ezek 44:22,23; Rev 14:4). A betrothal may only be terminated with the unoffending party free to marry another on account of prior intimacy with an illegitimate partner (cf. Matt 1:18-20; 5:31; Deut 22:13-21) The context for this process, which Yahweh illustrates through his love-relationship with Israel, provides a pure and historically proven means of courtship established on a promise of fidelity (Hos 2:19,20) which creates a lasting foundation of nuptial love and reciprocal trust.
Within the context of marriage, "whoever divorces his wife and marries another commits adultery against her, a woman who divorces her husband and marries another commits adultery against him" (Mark 10:11,12) "and everyone marrying her who is put away from her husband commits adultery" (Lk 16:18). "And when a man has taken a wife...and find in her an issue of unchastity...and write her a certificate of "putting away"... If when she departs she becomes another man’s... then her former husband which sent her away may not take her again to be his wife, after that she is defiled; for that is an abomination before Yahweh, and you shall not cause the land to sin which Yahweh your Elohim gives you for an inheritance." (Deut 24:1-4) "But if her husband be dead, she is loosed from that (aspect of) Torah so that she is no adulteress, though she be married to another man." (Rom 7:3b) "The wife is bound by the Torah as long as her husband lives, but if her husband be dead she is at liberty to be married to whom she will; only in Yahshua" (1 Cor 7:39) Therefore "Let not the wife depart from her husband, and if she depart, let her remain unmarried or be reconciled to her husband: and let not the husband divorce his wife (1 Cor 7:10)
Forgiveness from past sexual sins remains possible, but as with any transgression, only upon turning from that sin in complete repentance, and abiding by the restrictions which Torah proscribes (e.g. celibacy, etc.) for the “state which you are now in”. Defiant (un-turned / un-repented, or repeated) adultery, polygamy, homosexuality, etc. leaves no option other than "purging the evil from Israel" (Deut 13:5, 17:7,12) by disassociation and removal of that immoral individual from fellowship among the assembly of the Mashiyach (1 Cor 5:11-13, Rom 16:17, Titus 3:10,11). "Therefore put away from among yourselves that immoral person." (1 Cor 5:13)