Before he was Mr. Fantastic, he was Reed Richards, just an ordinary guy without the superpowers he acquired once he transformed himself into Marvel Comics' version--okay, ripoff--of Quality Comics' Plastic Man.
Reed was hot for Johnny Storm's older sister, Susan, but she wouldn't give him the time of day because, according to locker room talk, he had not only an undersize, but a miniscule, penis. Sue was interested only in manly (that is, well-endowed) men. Reed just didn't fit the bill.
In an attempt to impress Sue, Reed began to mess around with "unstable molecules." As a result, his body became extremely pliable, and he was able to elongate any part of himself--that's right, even that part of himself--and, as a result, he became, as far as length, if not girth, is concerned, the manliest man of the The Fantastic Four and of all other superheroes. Indeed, it was none other than Sue who nicknamed him "Mr. Fantastic" because of Reed's new-found virility.
A former friend of Reed's, who now called himself Dr. Doom, became Mr. Fantastic's arch nemesis, for Doom had also taken a fancy to Sue, and, alpha male that he is, he wasn't about to take his defeat lightly. Reed would have to pay--and pay--and pay.
Nevertheless, in 1965, Mr. Fantastic married Sue, who, in his shadow, was so ignored by her husband's fawning fans that she began calling herself The Invisible Woman.
Between them, they spawned an offspring, of sorts, a boy, of sorts, whom they named Franklin. A product of both mutant sperm and a mutant ovum, Franklin quickly developed into a monstrous freak and had to be put down (although readers were led to believe that he had been given up for adoption and was taken in by a powerful witch resembling Agatha Christie.)