Can the gender be unspecified even though the models are very masculine?
[Moderator moved the thread to the correct forum]
The gender is informative only. It's, however, interesting information for searching the database.
So yes, you can keep it unspecified, or you can put in any gender that is appropriate for the model.
If your models are recognizable, in that a model release is required to submit the image, I suppose you should ask the models themselves how they prefer to be labelled.
Read Adobe's guidelines on this page:
"Engage in respectful conversation with your model and give them the opportunity to provide information about how they self-identify. Keep in mind that demographic information such as gender and ethnicity must be accurate, both in your model releases and in your assets."
Many Buyers search for assets specifically by gender, so to omit that field might limit where your assets show up.