As stated previously, this site follows the rulings of Rabbi Yoel Schwartz of the Jerusalem Court for B’nei Noach.  As regards marriage, he states the following (I added paragraph headings and brackets for clarification):

Marriage:
Every man should marry a woman, and every woman should marry a man since it is written “that it is not good for a man to live alone by himself.” Marriage brings about a partnership between two people and increases the population of the world. In marriage a Noahide will participate and contribute in the building of the world. Marriage is initiated by the couple’s decision to live together as husband and wife followed by their sexual union. It is worthwhile and enhancing that the two hold a formal marriage ceremony and a marriage feast.

Divorce:
A married couple can divorce even if both don’t agree to the divorce. If the husband sends his wife away from their home declaring that they are no longer married or if the wife leaves their home with a similar declaration they are no longer married. However, Noahides should abide by the laws of the state in which they live in marriage matters. They should therefore also follow any procedures that are required by the laws where they live so that they should be considered divorced also according to these laws. After the divorce, both partners are free to remarry. The marriage also ends if one of the two dies.

Forbidden Relations
According to the Torah, a married woman and another man are not allowed to have sexual or any intimate relations with each other. A man, on the other hand, is allowed to marry more than one wife or concubine. Since the l0th century the Ashkenazic Jews generally have only one wife. Today there are also civil laws in many countries against bigamy, which are to be followed as law for the Noahide.

A person is not allowed to have sexual relations with his mother, sisters, maternal aunts from his father or mother’s side, his father’s wife even after the death of his father, daughter, granddaughter, daughter-in-law, mother-in-law and her mother, his wife’s daughters and granddaughters. A person is not allowed to have homosexual relations (man with another man or a woman with another woman). Also a person is not allowed to have sexual relations with animals. This is bestiality.

Safeguarding from Transgression
Rape by a Noahide is a capital offense. There are opinions that, regarding people who are forbidden by the Torah to have sex with one another, the Torah prohibits any physical contact between them that is liable to be stimulating. This would include kissing, hugging and social dancing. It is nevertheless permitted for people to kiss their offspring.

It is worthwhile for men and women to minimize their mingling with one another. For this reason, youth groups and social activities should be organized separately for boys and for girls. In particular, when groups come together for prayer, etc. the men and the women should sit separately.

Proper Sexual Relations
The purpose of sexual relations is to increase the human race and to strengthen the ties between the married couple. According to the Torah, a husband is called to gratify and satisfy sexually his wife. The same holds true for the wife toward her husband. All this is to strengthen the ties between the married couple. God created men and women with sexual appetite and chemistry. Sex is a blessing from God, and it is not sin in the bounds of permitted sex under the seven laws of Noah.

One should not read pornographic literature or watch pornographic movies. As women who are dressed inappropriately can stimulate men easily, women should dress modestly.

Having Children
Most authorities are of the opinion that Bnei Noah are not obligated any more to have children. There is a minority view that they are. According to this minority view and the view of Nachmanides, masturbation is prohibited. Also, according to them [the minority view] only the female is allowed to use contraceptives. The ones that would be permitted are foam or pills or similar methods.

Timing of Relations
It is an ancient custom of Israel to be careful and not to have sexual relations when the woman is having her menstrual period. An Israelite married woman is not allowed to have sexual relations with her husband for seven days, starting from the first day of her menstruation period. After the seven days she must go to a ritual bath, a mikvah, before she resumes having sexual relations with her husband. A Noahide is not called to do so, but it is worthwhile for the couple to abstain from having sexual relations during the woman’s menstrual period.^[1]

[-iHN note: Western medicine is disputed on whether sexual relations during menstruation is healthy or unhealthy.  Chinese Medicine advices against it, as it causes 'Qi Stagnation' in women.  Of course, Torah laws are not rooted in health concerns, despite often leading to better health.]