4. No Inappropriate sex - ... but simply it means only 1 man + 1 women in marriage is allowable by laypeople.
Since this is the Vajrayana forum I would encourage to refer to the corresponding tibetan sources which state things a bit more elaborated and different and thus avoid misunderstandings that may be caused by this overly brief statement.
"The Fourth precept is not indulging in sexual misconduct. If we are married, this means having no sexual relations other than with our life partner. In general, the precept means not having relations with someone protected by the Dharma, someone protected by belonging to another, or someone immature or unprepared. Someone protected by the Dharma would be a person following the sojong or precepts for a day, or who is living up to certain precepts for several days, or a monk or nun who is following lifelong precepts. Someone belonging to another would in general be a married person. In marriage, each partner has promised to be faithful to the other, so that would be interfering with a faithful relationship. A person may be immature or unprepared because of age or because of some sickness that could be aggravated by a sexual relationship. This also refers to the use of force; for example, certain kings might use their power to force a woman to surrender to their wishes whether she is willing or not."
-Khenpo Karthar Rinpoche, Dharma Paths
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OM TARE TUTTARE TURE SOHA
"The world is dark when you're depressed; your thoughts have the power to invent your world." -Courage Wolf
"It is more important to be kind than to be right."
(I acknowledge I do not follow the quote above this, that is why it is there! so I will be reminded every time I post!