15Copper Blood and Hybrid Children
The fact that humans and Vulcans can breed at all should be impossible, due to how fundamentally different their blood works.
One of the main ways in which Vulcans differ from humans is that their blood is copper-based, while human blood is iron-based. This is reflected in the fact that Vulcans have green blood, which is a trait that they share with the Romulans. The green blood also results in bruises
that have a green tint.
It's possible for humans and Vulcans to breed and create hybrid children (like Spock) but the canon has differed on whether this can be done naturally or whether it requires a degree of genetic engineering to pull off.
Star Trek usually hasn't stressed over the details concerning inter-species breeding, but the Vulcan biology has been defined in such a way that any sort of hybrid child should be impossible.