After all, one cannot discriminate against an employee because of marital status. True, but one, most relationships don't end in marriage and, two, the anti-nepotism policy is not against marital status, but against conflicts of interests or perceptions of favoritism. There are many reasons for having such a policy. Here's some real-life examples which emphasize the need for the policy. Woman is Accounting Manager.
Man is sales rep.
His travel expenses are always paid first and, when it turns out that he's cheating on his expenses, both were terminated. High tech company. Programmer and programmer begin a relationship which fails after three months. The relationship not only screws up an entire project that the two were working on, but disrupts the work of others. Well, it was affecting his work, and his work was key to a new product.