He was speaking at a Mothers' Union sermon and told those gathered that traditional family values are a 'myth'.
He said: "It is so important to remember that the golden age mythology of stable Victorian values was just that, mythology.
"Family life was under great pressure, especially in the poorest parts of the country.
"Mary Sumner acted out of concern for a terrible situation, in which children were not nurtured, women were at risk, households were not stable and the church was not doing very much about it, other than preaching."
He said that new family structures are normal and here to stay "whether we agree or not."
Mothers' Union was being addressed at Winchester Cathedral to celebrate its 140th anniversary. The archbishop singled out the organisation and praised its aim of "supporting family life."
Justin Welby spoke of his own experience, having a non-conventional family structure.
His parents had divorced when he was young and he recently learnt his father was not his biological dad.
"I know from myself that there is nowhere I can take my families as safely as around the table in the family.
"And I know having grown up in a different environment, a different sort of household, what a gift of grace that is."
The leader of the church of England told those gathered that change is not bad, according to The Telegraph, but emphasised that many Christians are "living in a culture that they have not yet begun to come to terms with."
He said that the same challenges apply to all families and that people still have a "need for reliance on God."
Mothers Union says it looks "forward to identifying further ways in which we can support the work of the Church going forward."