"Nullity" regards the validity of the wedding, or the entrance into the marriage.
To take an easy case, consider the case of two close relatives being married. Their marriage would be null, regardless of the number of their children or the duration of their domesticity.
Marriage is a natural as well as sacramental institution. Only marriage as a natural institution is relevant to civil law in a secular State.
A natural marriage is a lifetime commitment between a man and a woman for their mutual care, and the begetting and raising of children.
So a null marriage would include at least some of the following conditions:
- Failure to intend a lifetime commitment at the time of the wedding.
- Unwillingness to ever beget children at the time of the wedding.
- Failure to consumate; impotence at the time of the wedding.
- Mental incompetence at the time of the wedding.
- Etc.
Now, there are legitimate reasons for divorce, abuse being the most obvious. In such cases, the determination of fault should be paramount in awarding custody of children.
- Failure to intend a lifetime commitment at the time of the wedding. Easily, instantly claimed.
- Unwillingness to ever beget children at the time of the wedding. Even if children are begotten? Again, easily, instantly claimed.
- Failure to consumate; impotence at the time of the wedding. So, Stephen Hawking can't marry?
- Mental incompetence at the time of the wedding. Based on whose assessment? Define "incompetence." That disqualifies a huge segment of society for marriage.
- Etc. Exactly. "Etc." is the all applicable excuse.