By cuckold marriage I assume you're more specifically referring to black males with white, married females?
A cuckold marriage, by definition, is no more than one spouse having sex with other partners. Pretty cut & dry. Sometimes the spouse is aware and even encourages their partner, but in most circumstances the partner that is sexually active with other partners is "cheating" and/or having an affair, and the other partner is not the least bit aware. In that regard, lots of couples are choosing to have secret or personal flings, and think little or nothing of doing it.
In general, infidelity is a growing concern for marital relationships because there are so many more outlets & resources now that encourage & support it, than in the past ... chatrooms, Skype, websites such as this one. It might be noted that marriages are on the decline as people simply don't wish to take the time to properly bond and commit.