I don't know beyond a shadow of a doubt, but my gay friend said the penis was larger than his own when erect, ejaculated, and he could have intercourse with the vagina.
Also, it's a well known fact that some of these people can have sex successfully with both sexes. A little research on the topic makes that clear. You can also see this from the great variety of pictures. In addition, some other animals are entirely hermaphroditic, such as some species of snails.
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So, once again, who should these people marry?