Next time you see your husband, look him in the eyes and say the following.
The platypus presents absolutely no problem for evolution.
All proto mammals lay eggs. The echidnas, for example, is another early mammal that lay eggs. Why? Because proto mammals evolved from reptiles. While other mammals evolved to live bearing, the platypus along with other proto mammals never stopped laying eggs. Why? Because Australia was geographically isolated from the rest of the world very early on in the evolution of mammals.
Again, there is nothing puzzling about the platypus. It is a proto mammal evolved to live in a relatively small niche. The "duckbill" is absolutely NOT a duckbill. It is wide mouth with leathery skin that resembles a duckbill. This is a perfect example of convergent evolution (if you don't know what this is, I'll explain). This is primarily an aquatic mammal that hunts for prey in the muck in the bottom of streams. Other than that, this thing is all mammal.
Just look at the marsupials in Australia. Again, the continent became isolated fairly early on in the evolution of mammals. With really no other competition, these early mammals radiated into much of the same niche role as found in other parts of the world. There are the herd grazers, the predators, the ground hog type rodent roles... basically just a perfect example of adaptive radiation.
Disclaimer:
Occasionally, owing to the deficiency of the English language, I have used he/him/his meaning he or she/him or her/his or her in order to avoid awkwardness of style.
He, him, and his are not intended as exclusively masculine pronouns. They may refer to either sex or to both sexes!