First off, he looked at his big sis being miserable as a spouse and, since his married friends no longer had time for his feckless carousing, assumed sight unseen that marriage made people miserable. Only exposure to other married people after his own wedding opened his eyes to the truth that Elly was born a miserable misfit who could only see bad things. This probably caused him to regret dragging his feet.
Next, there is his mistaken belief that Elly and John are normal parents with normal children. Since most of the married couples with kids he knew didn't let him intrude on their lives because his carefree lifestyle was a damned disruptive nuisance, he doesn't know that Elly and John are terrible parents to weird kids. While he sees a lot of examples of normality, only his having kids of his own would have taught him what a failure his sis is.
This leads us to my subject: his fear of holding down a mortgage because he thinks it means becoming frustrated and old like his parents and sister. Phil isn't aware enough of his family politics to realize that most of why his folks are anxious and morose is that they have to worry about pulling a crazy woman out of a hole so he thinks that home ownership is why they're upset. If Phil were to finally realize that Elly is The Amazing Colossal Human Clusterfuck, he'd ruefully smile as he looks back at a life of limiting himself for a dumb reason. As it stands, he wished that he could have enjoyed the cute little home he did get but between the crazy old man with the barking dog of scaring off the Avenging Black Hordes and expropriation so that his cute neighbourhood could be turned into parking for Wal-Mart and The Gap, he's sort of screwed. Now, he enjoys a condo in Westmount with Georgia vaguely wondering what would have happened had he had a normal sperm count.