"Yes, Bleys?"
"After reading your chronicles, it was brought to my attention that no one in the family has ever given you the speech about the facts of life."
"Err, Bleys, I've slept with dozens of women. What do you mean?"
"Well, you talk about our reduced fertlity, for example."
"Yeah? I've got 2 kids I know of, in 600 years. Seems pretty low to me."
"Well, that's just it, Corwin, we of Amber can control our fertility."
"Err... we can?"
"Yes, exactly."
"And no one ever told me?"
"Well, we all thought you knew."
"600 years I've been shooting blanks because no one thought to tell me?"
"Everyone thought someone else had told you."
-- Bleys tells Corwin about the birds and the bees.

During the course of the first five books, Corwin mentions that the family is not know for its stunning fecundity, a fact borne out by Oberon's record of mere 47 children. That seems like a lot at first glance, but Oberon's lifespan was measured in thousands of years. Assuming he lived a nice round 2000, that's one child every 40 years or so, which is surprisingly low considering how lusty Oberon reportedly was. Corwin spends upwards of 400 years in Shadow Earth and doesn't seem to produce a single child. So what is it that keeps Amberites from having children?

The obvious answer is just that the Amber genetic line isn't so good at producing children. That, however, is boring. So here are some other possiblities for the hows and whys of the low fertility of Amber's royal line.

And there you go. Nine different potential explanations for Amberite reduced fertility. Now you can have a reason for it in your campaign, and thus a way to "fix" the problem if you should need.