What does having a child have to do with having a healthy relationship? Many married couples are very happy without ever having children. Many couples choose to have children and that's fine, but that isn't what builds a healthy, strong relationship. In fact, that mindset can be dangerous. A lot of women with abusive husbands think if they have a child together - he'll change and become more loving etc. Other women use pregnancy to "trap a man" and the child is then born to parents who didn't have children for the right reasons.
A relationship should be healthy first and then, and only if both people want children, should having kids be discussed. Children aren't for everyone. Sadly, many people feel pressured by society to have kids they really don't want. The result is many suffering children in this world due to selfish adults who don't think about the long term ramifications of their actions.
Can children strengthen the bond of an already healthy relationship? sure. Should they be used for that purpose? absolutely not.