You asked about whether it's common for secondary partners to have children together. I prefer to word it that way versus saying "is it normal for secondaries to have children with the primaries" because I think of those terms as describing relationships, not individuals -- she and your husband have a secondary relationship. And, since you're looking at things from an intentionally heirarchical viewpoint, the idea is that it will always be a secondary relationship, never a primary one.
From that perspective, not only would I say it doesn't seem to be common, I would also say that it probably should NOT happen if you can avoid it (accidents do happen of course, so it's not a bad idea to talk about what you would do in that event, but that's a different flavor of conversation).
Raising a child together is a huge, life-changing commitment, and if you're not prepared to live your lives together for the next 18 years, or at least give it your very best attempt, you're accepting from the start that the child won't live in the loving embrace of both of its parents. I'm not saying that single parent homes, or homes where one parent is a step-parent can't be amazing, of course they can, but shouldn't the first choice be to give a child the chance to live full-time with both biological parents if possible???
To me, raising kids together (assuming you want kids period) is part of what defines a primary relationship.
The major players. Me, under-30 bi female. Gia, girlfriend of 3+ years. Clay, new boyfriend/dom. Davis, ex/"it's complicated."
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy, Clay's partner.