Looks like someone left off the "when a Mommy and Daddy love each other very much" part of the Where Babies Come From explanation for Alexander.

"Welllll," Scully said carefully. "Obviously. But. If Cassima loves you as much as you love her... and, well. You're the same age, so she's not too young for the proceedings. She might, after all-- welcome the idea." She grimaced. "She definitely would if she were here this week. Waiting for the wedding would be entirely problematic."