Even the X-men,though of a terrestrial origin ,are still an advanced species as compared to humans but the possibility of mutual sexual attraction between the mutants(homo superior) and baseline humans(homo sapiens) is still plausible as inter-species breeding has been observed(mostly in captivity) between verisimilar members of the same biological family such as between dogs and wolves and lions and tigers.But,in the case of mutants possessing profound physical manifestations of their mutations(eg.Beak of the X-men) which would also include changes to their gonads,this will not be the case.
Maybe,comic book characters and their romances are to remind us that love is all transcending and can unite even wholly different species.And considering the many alien abduction claimants in which the subject has claimed to have had sexual intercourse with one of his/her captors,one wonders whether this holds a key to superman's problems.