 Why does Peter insist that Lorelei not come along for the mission?
They have treated us precisely as we, in less enlightened days, might have treated a newly-discovered race of lower animals. "I have no fear that the people of Earth will fail to meet this challenge, for there is no alternative. It was only two stories down the moving ramp to Lorelei Cooper's laboratory. Peter took it in fifteen seconds, running, and stumbled to a halt in front of the door marked "Radiation." It was a street scene in the Science City of Manhattan, flooded by the warm spring sunshine. This isn't like a war, where the noncombatants' morale has to be kept up. There aren't going to be any noncombatants, this time.
 He knows she would mutate as well, and he wouldn't be able to handle that and put the mission at risk.
 He knows he will mutate when he leaves, and he can't stand the thought of her seeing him like that.  It's too dangerous for her to go as a woman. She doesn't have the same odds of survival.  He knows she would mutate as well, and he wouldn't be able to handle that and put the mission at risk.  He knows that they'll be reunited, and promises to come back.