Because kamma can be comprehended and ended without recourse to a God or gods however one is free to pick favorites to assist one onward to liberation from dukka.
Actually, illicit sex in Theravada for a lay follower is the most liberal you will find amongst major religions.
" (3) Abstaining from sexual misconduct (kamesu miccha-cara veramani)
He avoids sexual misconduct and abstains from it. He has no intercourse with such persons as are still under the protection of father, mother, brother, sister or relatives, nor with married women, nor with female convicts, nor lastly, with betrothed girls.
The guiding purposes of this precept, from the ethical standpoint, are to protect marital relations from outside disruption and to promote trust and fidelity within the marital union. From the spiritual standpoint it helps curb the expansive tendency of sexual desire and thus is a step in the direction of renunciation, which reaches its consummation in the observance of celibacy (brahmacariya) binding on monks and nuns. But for laypeople the precept enjoins abstaining from sexual relations with an illicit partner. The primary transgression is entering into full sexual union, but all other sexual involvements of a less complete kind may be considered secondary infringements.
The main question raised by the precept concerns who is to count as an illicit partner. The Buddha's statement defines the illicit partner from the perspective of the man, but later treatises elaborate the matter for both sexes.
For a man, three kinds of women are considered illicit partners:
(1) A woman who is married to another man. This includes, besides a woman already married to a man, a woman who is not his legal wife but is generally recognized as his consort, who lives with him or is kept by him or is in some way acknowledged as his partner. All these women are illicit partners for men other than their own husbands. This class would also include a woman engaged to another man. But a widow or divorced woman is not out of bounds, provided she is not excluded for other reasons.
(2) A woman still under protection. This is a girl or woman who is under the protection of her mother, father, relatives, or others rightfully entitled to be her guardians. This provision rules out elopements or secret marriages contrary to the wishes of the protecting party.
(3) A woman prohibited by convention. This includes close female relatives forbidden as partners by social tradition, nuns and other women under a vow of celibacy, and those prohibited as partners by the law of the land.
From the standpoint of a woman, two kinds of men are considered illicit partners:
(1) For a married woman any man other than her husband is out of bounds. Thus a married woman violates the precept if she breaks her vow of fidelity to her husband. But a widow or divorcee is free to remarry.
(2) For any woman any man forbidden by convention, such as close relatives and those under a vow of celibacy, is an illicit partner.
Besides these, any case of forced, violent, or coercive sexual union constitutes a transgression. But in such a case the violation falls only on the offender, not on the one compelled to submit.
The positive virtue corresponding to the abstinence is, for laypeople, marital fidelity. Husband and wife should each be faithful and devoted to the other, content with the relationship, and should not risk a breakup to the union by seeking outside partners. The principle does not, however, confine sexual relations to the marital union. It is flexible enough to allow for variations depending on social convention. The essential purpose, as was said, is to prevent sexual relations which are hurtful to others. When mature independent people, though unmarried, enter into a sexual relationship through free consent, so long as no other person is intentionally harmed, no breach of the training factor is involved.
Ordained monks and nuns, including men and women who have undertaken the eight or ten precepts, are obliged to observe celibacy. They must abstain not only from sexual misconduct, but from all sexual involvements, at least during the period of their vows. The holy life at its highest aims at complete purity in thought, word, and deed, and this requires turning back the tide of sexual desire"
"The Noble Eightfold Path: The Way to the End of Suffering", by Bhikkhu Bodhi. Access to Insight (Legacy Edition), 30 November 2013, http://www.accesstoinsight.org/lib/auth ... toend.html