You used to say that you wouldn't have sex with Jeff if he was the last man on earth. Well guess what. The apocalypse just happened and Jeff is the last man on earth so it's up to you having sex with Jeff if you want the human race to continue.
Jeff will sit down on a piece of the broken Statue of Liberty and feel bad because there's still rejection even at the end.
"But... Then we're just handing it all over to the machines and the intelligent rodents," Jeff will beg.
"Sorry," tell Jeff. "I meant what I said at that Christmas party. Not if you were the last man on earth. How would it look? Everyone heard me say it."
"But they're all incinerated," Jeff will say.
Tell Jeff that you can't in good conscience unleash upon the planet the kind of human race that would be born from a sex act between disparate castes of attractiveness.
"That's not a race I wanna be a part of," tell Jeff. "Sorry."
You and Jeff will continue to live together platonically, working together to keep warm and fight off all the stuff that's turned huge and/or smart thanks to radioactivity. Jeff will grow stronger and a little more attractive, and he'll comfort you sometimes, but it's still not clear whether you'll ever be able to drop your superficial dating rules and finally realize that you two were made for each other since you have no choice, making you two the last will they/won't they couple on earth. Except this time we all hope it will be will they not because it would make us feel warm and happy, but because it's the only way there will be future generations who might tell our stories and carry on our traditions. Also, for there to be future generations not only would you two have to have sex but so would your kids. With each other. Sorry.
Happy Last Will They/Won't They On Earth Day!