A long time ago I did. I lasted a long time and we had a child together. I don't regret a thing. I love my husband dearly, he does not worry about where my heart is. I think he appreciates not having the drama that comes with blended families. They like each other. I am really close to the ex's new family and it is genuine. We celebrated the kids birthdays together or take in some shows. It did go thru an awkward stage after the split years ago, but I think had he not have been my close friend first, we would not be how we are now.
As a general rule, I regret very little if anything in life anyway. I find that the time is better spent elsewhere, but honestly, I wouldn't change this part at all.
If you really sit down and think about it, if you never date him and then married someone else, you would have pretty much the same kind of relationship with him that I have with my ex now with out the child. Your married life would keep you busy but you may get together for special occasions and keep up with his family.