As the title suggest, when in the image, only a penis is present as a representative of the male genitalia - under the |tag what you see| idea, i usually don't see why we should add the "1boy hetero" combo.
Though, in some cases you can gather from the image that the other person is supposed to be male, (post #3333301 for instance prob has the admiral) in most cases, it can just be a penis, or a futa, or a monster, it doesn't really matter since they are not actually present in the image, just the penis.
In the case of a disembodied penis especially, considering the fantastical elements of anime/manga, it can be just a penis with no person attached, as far as the "lore" of the image is concerned, and for tagging purposes.
This has been a topic particularly interesting to me because people keep re-adding those tags on some of my tagged images, so i wanted to get a feel for what the community thinks.