This practice may seem unusual to common Buddhists in other part of the world but in Japan it had been practice since 1872 when the Meiji government decreed that Buddhist monks and priests (but not nuns) should be free to marry if they chose to do so.
"Temple families" soon became commonplace and the administration of temples and monasteries became family businesses, handed down from fathers to sons.
However, this kind of practice is not new in Buddhists world. Among Tibetan Buddhism there were a time when monks were married too. Now Buddhist Lamas (teachers) for the same reason, i.e. to carry on the lineage, but they are not monks. They have to return their monks vows to do that. One of of the more popular teachers is Chögyam Trungpa Rinpoche (February 28, 1939 – April 4, 1987).
He disrobed to marry English girl, Diana Pybus and had children with her, who are later recognised as Rinpoches. Before that he also had a child in India, this child is later known as Sakyong Mipham Rinpoche, an incarnation of the famous "Mipham the Great".
Rinpoche, the eleventh Trungpa tülku, was a Buddhist meditation master and holder of both the Kagyu and Nyingma lineages. He is a recognised "tertön", he was the supreme abbot of the Surmang monasteries; founder of Vajradhatu and Naropa University and establishing the Shambhala Training method. He was the prominent master that brought Tibetan Buddhism to the West.
So for a high lama or highly attained teachers, the practice of getting married and having children is not a negative action. As a matter of fact, it is for the benefits of others, that is for the continuity of the lineage. In the case of Japan's Buddhist monks, we can say that it is what needed by their society.