I have an issue with people, who fall "in love" quickly. Likewise people who live together (or marry) quickly. I've known so many couples split, that I'd take listening to the vows from a couple that have stood the test of time, more seriously.
DH proposed when we had been together for 4 months and we got married the following year. 20 years and several children later, we're still going strong. Unlike the couple of the only wedding I've ever been to that already had children together. They lasted less than five years, sadly.
To be fair, the friends I mentioned only got together because she became pregnant after a ONS. I admire them for making a go of it, and it's sad that it didn't work out.
I guess we're a bit unusual as pretty much all our friends got married first, lived together as a married couple for a few years and then had children. And looking back at the many weddings I've been to over the years, only three have so far ended in divorce, including the one above. So I certainly don't think getting married before having children makes divorce any more likely than doing it the other way around. And in my personal experience, co-habiting couples who have children together without being married seem more likely to split up. Very few of the single mums that I know were married, although most of them were in a committed relationship with the father at some point.
I agree with OP in that while I'd be very happy and emotional for the couple at any wedding, I couldn't get quite as excited about one where they had lived together for years and had children together. A wedding for me is not just about the couple but about two families coming together and each family welcoming a new member. In the case of established families, this has presumably already happened.