The program filters for gender if it only shows a user of one gender the friends with the opposite gender. To filter for any gender at all, they have to ask Facebook's API for gender data about users. It's easier not to ask at all and just offer all users. Instead it checks that gender of user does not equal gender of list presented. The computer doesn't maaaaaagically know "boys should want to see girls or it's icky": that's in the app's code. If they plan to add it later, that's only because non-bros made a stink. It doesn't matter if you don't want to see the evidence of breeder privilege. It's there. I spend enough of my life dealing with code to know it took them effort to get this non-toggle option.
More like the app would be a lot shittier if i had to scroll through most of the dudes on my friends list just to protect the sensibilities of a few. It kind of defeats the whole purpose of the app if i'm not even getting to see anyone I would bother rating on sexual attraction.