I know it feels kinda patronizing, but I think the reason there is a lack of male portrait support is because the game doesn't seem to be advertised that way. The pics in game (like the slave market background) as well as the images in the game's page show only women, so it doesn't feel like men are involved at all. Maybe make it more clear that its supported will attract more artists to make more male portraits.
The core pictures do seem to focus on female slaves. Probably cause that's the core market to target, and perhaps it's a preference of the dev or the artist he paid. Though the portrait packs are mostly just pictures players have found online. Not many artists contributing directly here. So it appears the demand for male portraits hasn't been enough to provide a supply.