Gralson and I don't have any boundaries about when we meet other people. When I started dating Auto, Gralson was working out of town a lot, so there really wasn't an opportunity for them to meet until Auto and I had been dating for a couple months.
On the flip side, Auto and her husband have a policy that he gets to meet her new dating partners before any "sexy time" happens.
Auto and I have a very uncomplicated relationship. We've never talked about meeting new partners, but we always seem to work things out pretty easily, so we would play it by ear.
For me, it would feel presumptuous to arrange a "meet my other partners" before I knew the relationship was actually going somewhere. Conversely, if someone insisted I meet their spouse before the second date, it would feel like they're expecting something out of me and the relationship. That kind of pressure turns me off and I would probably choose not to see them a second time.
To me, a "rule" about when to meet other partners is weird. Every person, relationship, and situation is different. How could you have a one-size-fits-all response? I might meet one person and be unsure how I feel about them, so I would want a second date to get a better picture. Someone else, it might be instant chemistry, and I would want them to meet everyone asap so that I can get their impressions before plowing full steam ahead. So the most reasonable approach is "after you know there's something there, but before it gets too serious." But that's kinda wishy-washy for a "rule."
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker