Out-of-wedlock births threaten a society's social cohesion for sure. This is because people come into the world without a loving mother and father that work in unity to raise their child. It creates stressed out children and stressed out adults. It also creates more dependency upon welfare. People on welfare tick off taxpayers. This instantly creates division in people.
Children born out-of-wedlock are no different from any other children. Their parents simply do not have a paper saying they are married. If the father or other family members support the child, then they will have the same family group that other children born to married parents have. There are many children born to married parents who do not have two parents due to death, divorce, desertion, or just unconcern. The lack of concern about our children is what threatens social cohesion.
Out-of-wedlock births do not threaten society's social cohesion. This event occurs so often now that it does not have threaten cohesion and many bond over this. As long as the parents continue to love the child and raise them to respect one another and be thankful for their opportunity at life this will not threaten societal cohesion.
I do not believe out-of-wedlock births threaten a society's social cohesion. Marriage is something associated with the state (in legal terms) and religion (in terms of tradition). There is no law that says absolutely that a couple must be married to have children and in today's secularized world, it is far less likely.
I do not think that out-of-wedlock births threaten a society's social cohesion. I think that while a lot of people may have problems with people who partake in such activities, I do not think that it is such a problem that it would negatively affect the society as a whole.