A few reasons I could think of ...
May I ask why you think that Sui Yangdi might not have wanted to have more children? Normally, Emperors would want to have as many children as possible to ensure that there will be more than an adequate amount of offspring to pass on the family bloodline.
(1) Too many sons, especially capable sons, translate into rivalries among ambitious siblings. Yang Guang himself should know.
(2) If Yang Guang liked the ministrations of these particular ladies, their getting pregnant would inconvenience him for quite a while, at least.
I think Centaur was correct - it was the norm for later dynasties.
Hmm, I am really confused regarding the “keep” or “discard” issue! I mean it is quite easy to understand that the Emperor would most likely want to keep the baby. However, what if the Emperor chose to “discard” this pregnancy? Would the concubine or Empress be forced to have an abortion?
I believe that this protocol was much more famous in the Qing Dynasty compare to other dynasties; however, a similar procedure was mandatory to assure that all babies were sired by the Emperor himself. This would be the case for any dynasties.
As for the precise procedure, I really do not know it, but some possible interpretation according to some books I've seen, well, it is very undignified to the ladies, not to mention ghastly, horrifying, I don't want to describe the speculations but suffice to say it is a too horrible thing to even contemplate.