Nicholas II and Alexandra of Russia

[Sarabellum1976]

They had 5 children in total. First, four daughters, all about 2 years apart. Then, 3 years later, their son and heir, the Tsarevich Alexei. Then they were done, apparently. I have read nothing that would suggest that they planned on having more, or were disappointed when no more children showed up.

Alexei was noted to have hemophilia pretty early on - not exactly a "death sentence" but not a good indicator that he would live a long life. So my question is: Why did Nicholas and Alexandra apparently choose not to have any more children after Alexei? Surely, it could have only been a good idea to have a spare boy waiting in the wings?

Why did they think one hemophiliac boy would be enough to secure the future of their monarchy?