I don't see why changing the NPCs gender would change story or even dialogue significantly though, aside from changing pronouns and some terminology? The only NPC that I think it could significantly effect is Bell. That being said redrawing all the assets is still a lot of work, I just thought it was an interesting idea given there's already a toggle for only male/only female random encounters.
Gender pronouns, genitalia, and even sometimes clothes get mentioned in nearly every character scene. And that's not even considering context and intent within the dialogue. You would basically have to re-write absolutely everything in every sex scene. Between all courtable characters, that's hundreds of hours of work for a set of toggles.
I'm not saying it's impossible, but you need to understand just what you're asking...