The couple he wanted to invite, they'd known for years. They'd all worked at the same place and, after he left that job, both his wife (to be) and he visited the couple regularly. 

(Of course, the couple are fictitious and, for the sake of privacy, I'll refer to them as Roger and Natalie)

You'd be hard pressed to find a nicer couple and Michael saw no signs of there being any problem between the two women on any of the numerous visits made to the couple's house for tea. They'd both attended R & N's wedding.

Roger and Natalie had had three children by the time Michael and his girlfriend decided to marry and it was 'a given' that the the couple and their kids would attend the wedding, or so he thought.

Despite the fact he came from a small family and had very few people coming to the wedding, Michael's fictitious wife-to-be informed him, at the last moment*, that she didn't want Natalie to come. And that meant none of them.

This was an early red flag and, in retrospect, Michael admits he should have stood his ground, but he didn't. He was too agreeable. Maybe he would have been less so if a fictitious child wasn't already on the way.

Secondly, he regrets not, at least, talking to Natalie to get to the bottom of the issue. Twelve years later, Natalie told him something that would have been another red flag. It was obvious that Natalie could see right through the woman he was marrying and a conversation with her could have saved him years in time and tens of thousands of pounds in money.