plain and simple, if you ain't going to produce kids in a marriage then what is the point of the marriage? you can do everything else as just a friend so marriage isn't needed.
If we just cut to the chase... And there are usually plenty of threads talking about it in singles -- it's a basic fact of life that a lot of people want to have sex. And if they are abiding by conservative Christian principles, the only way to do so is to get married, whether or not they have kids (and we would all be millionaires if we all got a dollar for every time we're given the "It is better to marry than to burn with passion" reference.)
Of course, this also leads to the problem of people marrying too quickly or for the wrong reasons just because they want to have a sexual outlet, but that's a whole 'nother discussion.
There are a lot of single parents out there who don't want more kids, and would not be marrying to produce more children. Yes, they would be raising the children they already have, but they wouldn't be marrying to have any more. Many single parents also have grown, adult children (and/or grandchildren,) and are looking to marry but definitely not to have more kids.
While I understand that many adhere to the principle of "be fruitful and multiply," a Christian friend once pointed out that this command was given two specific times in the Bible -- once after Adam and Eve were joined together at the beginning of humanity, and once after the Flood and destruction of all but 8 people left on the earth.
In other words, these were two very specific conditions under which there was a need to fill and populate the earth.
If someone believes that marriage is only for the creation of children, maybe that's what God has put on their heart for them to do. But I've spent most of my life living around older adult populations, which has made me realize that the search for love and companionship has no age limits or boundaries. When my older relatives' spouses died, they all sought another companion, and it certainly wasn't to have children.
A while back, I met a lovely couple, probably in their late 60's, who had been married for about 2 years and still acted like newlyweds. Both had had major losses in their life, and neither was looking to find someone. The woman had put a vehicle up for sale, he showed up to answer the ad, and thus began the trip that landed them at the altar.
The husband also had an artificial limb. I don't know if it was from illness or military service, etc., but it made me think a lot about how this couple, with two lifetimes of experience, probably knew a lot more about accepting someone as they are and sticking with each other "for better and for worse" than many of us will ever know.