Biblically speaking, are married couples who are physically able required to have children, or can a couple come together in marriage for the sheer enjoyment of one another's company and building up of one another? In other words, can one marry and rightly choose not to have kids and/or take measures to be made unable to have children such as surgery or using protection?
While children may be a blessing and many couples who have them may believe their friends who don't are missing out, quite simply, there is no biblical command to have children. Of course we have the mandate in Genesis 1 to be fruitful and multiply; however, this does not apply today.
The New Testament assumes in many places that we will have families (e.g., Eph 5-6). However, it also teaches celibacy as both a gift and a viable option (Matt 19, 1 Cor 7). Yet there is a generality to many difficult passages: the Bible often speaks of the usual, not necessarily the exception.
One last thought: whether or not we as couples have children, we are called to be giving to all persons. Jesus had no children, but the children came to him and apparently felt comfortable with him. Whether or not we have our own children, nearly all of us are uncles or aunts, or "father figures" to younger persons.