I reread this part twice, and I had a completely different take on the specimen. Poor Kal just doesn't get the whole concept, and for all we know they are getting him to spit in a vial. I don't think he has the slightest idea of what causes a baby. It might be that Za-Ra doesn't have any more idea than him, so she thinks it is all on the level. The rest of New Krypton knows about the birds and bees, and will never believe that their Supreme Ruler and his wife didn't understand what to do.
I think also that as sheltered as Kal's upbringing has been, his "secret" that has him embarrassed is probably something minor that one of the nobles has convinced him is a major character or moral flaw. IMHO
Pat
