Second, they're demonstrably able to interbreed with us in numerous stories from various eras of Superman. Sharks and dolphins may have developed similar forms due to parallel evolution but they certainly cannot interbreed with each other just the same.
L. Sprague de Camp once said it was easier to breed with a geranium than it is with an E.T.
Here's one possibility related to future stories involving "Imaginary Story" future children and descendants of Superman: is it entirely possible that said life forms were created artificially, under laboratory conditions and then nursed in the womb?
This does not mean that Superman and his female girlfriends would not have real passion or physical intimacy; it means that intimacy will not result in a viable pregnancy. Perhaps what took place is Superman and Lori Lemaris or Lois Lane or Lana or who have you, married, made a decision to have children, and so through genetic tampering their DNA was combined, and after a surgery, was placed in the womb of the female to grow and gestate.
The technology for this is not entirely farfetched even TODAY; there was a recent article where scientists said they may be able to create babies for same-sex couples that are genetically related to both of them.