Lama Ole’s answer:
It depends. If you really think that this is your partner, then you have a certain responsibility to make him or her happy as best you can. On the other hand, you have a responsibility to help him or her grow and develop as a human being too. Ultimately, you need to weigh these two things against each other in such a situation.
It depends on one’s attitude, the way one thinks and experiences things. It depends on whether the partners have met on the level of “only you from now on to the grave” or if instead they have thought, “from now on, we are going to grow and develop side by side and manage the maximum possible part of the way together.” All this varies person to person. It is impossible to set fixed rules there.
With a jealous partner, for instance, one should indeed think seriously about whether an affair is of any use. On the other hand, if one’s partner thinks, “he knows what he’s doing” or “she knows what she’s doing” or “if she’s fine, I’m fine too, and if she falls in love with someone else, she’ll take more home and make everything new and fresh again.” In this case, it would be quite OK—always subject to the condition that the partners are healthy. But to do this, there must be a lot of trust between the people involved. If one goes to bed with somebody else, one goes to bed with all the partners that person has had since there’s something of all of them present. It is certainly a bigger thing.
One should always find grown-up partners and not small, dependent people with little surplus. It is not a question of morality, where we should think “bad” or “must not”—not at all. In Buddhism, the body isn’t considered something fundamentally bad, like in a few other religions. The body is considered a palace of light with 72,000 energy channels, all made of the nature of wisdom. The body is a tool to benefit and help others, to give them happiness and love.