Other social animals have cultural rules as well as us, jess. Forest-dwelling is irrelevant.
"One could argue that humans evolved within family groups so deciding to have a child who will not be part of such a group is flying in the face of biology?"
I don't think copulation, whether in 'approved' circumstances or not, is flying in the face of biology. Also, since we are such social animals, one can think of an entire society (in the past, hamlet, village) as family group. As chimpanzees, our cousins, do, for instance. In more primitive times this was necessary because life was more dangerous. There will have always been orphans as well as single parents for various reasons.
"I don't necessarily think children are burdens on society. We need people to have children who will grow up and do useful things and generate tax income in 25 years time don't we?"
Hmm, yes, but not all things that generate tax income are useful. Think celebrity culture and rich folk who avoid paying tax. There will always be some members of society who, through no fault of their own, cannot be as 'useful' in those terms (generating tax income) as others. They are not necessarily of less 'worth' to society – in some cases maybe more.