I wasn't there, but I was told about a priest who today tried to be so inclusive in his definition of what constitutes a family that even single people, living on their own without any children, counted as families. While single people can form part of the network of an "extended family", to consider them in themselves, as a family unit is a nonsense. It is first of all a complete contradiction of the ordinary meaning of the word "family"; and, in the second place, a single person who has given any thought to their vocation in the Church, will not see themselves in any way as having the same vocation/office in the Church as someone who is married. I, for one, have no difficulty as being "left out" from family definitions; I consider myself to have a completely different mission in the Church and in the world, and almost resent the idea that that mission will in some way be "forced" into being that of a family.
The first problem with the approach of this priest (and I suspect that it is not unique to him) is that it confuses totally what I would term the "catechetical moment" and the "pastoral moment" with regard to family life in a parish. Today's Feast of the Holy Family clearly provides an opportunity for a "catechetical moment" - an opportunity for a systematic presentation of Catholic teaching about marriage and the family life that follows on from marriage (but see below). This moment calls for clear and unambivalent teaching, presented gently but nevertheless clearly. Family should be defined in relation to marriage, and the openness to life that is part of the married vocation. Within a wider and more every day usage of the word "family" it is possible to acknowledge the situation of those who have children without being married, or who have children from earlier, broken relationships. But, within the "catechetical moment", the relative imperfection of these situations compared to a married family life can be spoken about without any attitude of condemnation. The "pastoral moment" belongs at another time, in responding to the needs of individual parishioners and their situations.
The second problem is one of public debate and policy making. The approach of including every type of household within the definition of "family" undermines any possibility of a consistent response from the Church to those who would re-define the family to include same-sex and co-habiting couples, without any reference to marriage as a constituting element of the family.
Somewhat as an afterthought, and observations to the contrary welcome in the comments box: but is the Feast of the Holy Family a feast that really provides opportunity for a full catechesis on marriage and family life? Seeing the Holy Family as a kind of "model" of what families today can be like has some merit; but, from a theological point of view, it does not seem catechetically strong to me, not perhaps reaching beyond the pious (not that that is in itself bad). In the homily I heard at Mass today, it was suggested that this "first" Holy Family should be seen in the context of a "second" Holy Family - that of Jesus on the Cross, Mary and St John at the foot of the Cross. The richly ecclesial interpretation of the mutual entrusting of Mary and John by Jesus at this moment gives access to an understanding of the Holy Family in the framework of the ecclesial understanding normally associated with the understanding of the sacramental nature of marriage.
[For an account of the pastoral possibilites of this feast, see Holy Family, the Octave of Christmas, at the Communio blog.]