Lama Ole’s answer:
First of all, it is important to understand that Buddha isn’t a creator god or a wrathful god wagging his finger at us with some type of moral judgement. Buddha simply wishes us well—that we live, die, and are reborn in a better way, and that we develop. To a very large extent, the Buddha stayed out of people’s bedrooms. This was the smartest thing he could do.
As a matter of fact, there is a lot of variation among people. From a cultural standpoint, there are many approaches to fidelity. In the West, the one-to-one model prevails: one man with one woman, one woman with one man, and they have a few kids. This model has generated the most solid society in the West. And indeed, it looks like it fits best for most people. But often, it can become a bit too tight; there is too much pressure and too many expectations, like a pressure cooker. In other countries, polygamy is practised, where the man has various women partners. But there are also countries, like Tibet, where women have several male partners.
In Greenland, the code of hospitality entails that the host allow the visitor to spend the night with his wife. That’s because the visiter has often travelled over ice for a few weeks, whereas the husband is always around. Of course it is a precondition that the wife agrees to this.
In Arab countries, by contrast, the women have to cover up their bodies since the men don’t want other people to see how beautiful or how ugly they are. Unfortunately, as a consequence, women’s mobility is limited to a high degree, and a lot of traffic accidents happen because they can’t see properly.
Between these models of freedom and limitations, different people find their way. To a large extent, this depends on karma. In most cases, we probably have the strongest karma with the one or two partners we were already together with in former lifetimes.
Incest was the only thing that the Buddha resolutely opposed. On this point, he was absolutely clear. But apart from that, one can live well as a Buddhist in nearly every society without getting into trouble. A sexual relationship that doesn’t cause harm is a good sexual relationship.
However, the moment that children join in, true responsibility emerges. Then we have brought beings into the world who depend on us. In this case, we should try to be a good family or at least have a good relationship with one another.