Open relationships might be an interesting model for some people, but it can become a toxic dark pit for a relationship. If both parties generally want and agree to an open relationship, it should be able to work, but not once you decide to start a family, especially if family time is encroached upon by one of the parties seeing another person. It could also become an issue if one of the parties feels that the other party is spending too much time away from home, and not sharing in the workload of the family.
The other factor can be jealousy. Even if someone allows an open relationship they may not like the idea so much once it becomes clear that their partner favors the other person.
The most important thing, however, is to figure out what each party wants or like before you get married. Do not wait until after the marriage to tell your spouse you want an open relationship! Go to GoMarry.com to find the means to ask the difficult questions before the wedding day!