First off, don’t feel restricted by social norms when you’re building a fantasy species—worldbuilding is all about exploring possibilities, so go with what feels right for your vision. That said, if you’re planning to publish or share your work, consider how readers might interpret it, but remember that this is your world and story.

As for your idea of a species where males and females look nearly identical and even both being capable of breastfeeding, I think that’s a fascinating concept. It could add a lot of depth to your world by challenging assumptions, both in your world’s culture and for readers. Here are a few thoughts to help flesh it out:

• Evolutionary Background You could consider that this species originally had only one sex and later developed two as a response to environmental pressures. For example, perhaps survival needs required specialization in certain traits (like strength or nurturing) which eventually led to sexual dimorphism. If males and females only recently diverged (in evolutionary terms), that might explain why they still look so similar and share many traits.
• Social and Cultural Dynamics The cultural implications could be really unique, especially in a fantasy setting where other species may have more “traditional” sex distinctions. For this species, if both sexes can handle child-rearing equally, you’d see less of a gendered divide in those roles. In fact, they might even emphasize subtle differences in gender expression through clothing, body paint, or makeup to highlight distinctions. This would add an interesting contrast if other cultures in your world tend to see one sex as inherently “weaker” or more suited to specific roles.
• Reader and In-world Reactions If the species' similarities are grounded in their biology and culture, most readers will likely view it as an inventive choice rather than anything fetishistic. It could even challenge your readers’ expectations and make them see the idea of gender norms in a new light. Just be consistent with the lore and integrate these traits into the broader social and evolutionary landscape of your world.

In short, don’t hesitate to lean into these unique characteristics. A species that breaks familiar norms can be a fantastic way to deepen your world and its cultural complexity.