The pattern has endured throughout garden history…
The boys come along,
They meet up with the girls,
Love is in the air,
Male Spinach seeds releasing pollen
And seeds are formed.
Spinach Seeds being formed near the stalk.
For further questions, go ask your Mom and Dad.
It came as a surprise to me that spinach plants grow up to be either male or female, and do not have both the male and female flowers on the same plant. The term for this is dioecious, meaning two houses. One way you can imagine this is having all the boys in one house (plant), with all the girls in another house (plant). As long as they are in the same area, seed will be formed.
Seed to Seed recommends having a minimum of 4 female plants and 2 male plants to continue the vegetable variety without any detrimental inbreeding. I laughed at this, because you cannot determine which is male or female until the stalks grow. So the basic rule of thumb is the more the better. As spinach requires a large isolation distance to retain varietal purity, small-scale gardeners may find it easiest to save seed from one variety per season.