And it's possible considering the current progression that Karen will return as a dude instead.
Something tells me you haven't made it very far in the main story.
You must have gotten the wrong impression, this is never alluded to nor explored in what I'm assuming is the Fiend hub section. There's only one futa shown in the fiend tribe and she was not made to be like that, but she just happens to be one. There is nothing more to it than that.
Also, you can play this game without doing any sex. This is called the pure route. You do the main story, but decline any sexual advances made to you and don't fail any skill checks like fights and such. It's meant for people who just want to play the game and enjoy the story. The gameplay is harder here because it assumes that you're not interested in the porn for now. It also tests your will much like the scenarios test Jenna's because giving in to sex in these cases is way easier gameplay-wise than doing things the hard way.
You can also pick and choose which sex scenes you want to go after in the main route, meaning you can do a full lesbian run if you choose. This is not possible in the slut route. Yes, there could be more lesbian scenes in the main story and there are more on the way.
This game was never meant to be a lesbian-only affair. It has all types of content, but the balance usually tips in the favor of straight-oriented content because that is just what we personally prefer.
I'm also not sure where you're getting the impression that the storefront heavily advertises girl-on-girl. Looking at the Steam page's screenshots, I see two purely lesbian CGs, and two with dicks. Not to mention these CGs weren't even picked with any thought other than they were the spiciest I could think of at the time. Also, we only recently started fully animating CGs, which is why a lot of the lesbian scenes are not like that. All of the animated ones you see in the early game are replacements from the older art style to the new one and since we were replacing them we decided to animate them as well. We're hoping to eventually go back and animate all of the CGs, but that may or may not happen.
Like... why plaster "you're a badass lesbian superhero, out to save your girlfriend!" all over the brand, if you're not allowed to do just that? xP
You can do exactly that if you decline all advances made to you and don't get distracted with the slut route. The main driving force for Jenna in the main story was and still is rescuing Karen.