An open marriage is fine if the other person is actually okay with it. This is rarely fully true. The only way that will ever be the case is when someone is acculturated from birth to believe in the goodness of polygamy (both polyandrous and polygynous) without any reason to believe in a contrary position. However, since too many societies exist where the mainstream accepted idea is one of monogamous love and because of the wide use of televisions, the Internet, and other media outlets it is unlikely that even the most Amish/old-school-lifestyle-type people can ignore the opposing thoughts. So, no, this is why I believe an open marriage is a bad idea. There is no particular way to measure the relative enjoyment of both partners as being "equal" when they commit adulterous acts (even with ground rules), and there's no 100% way to ignore the possibility that the reason your partner is seeking other people is because they don't love you as much or at all.