Can concur, I also did not get one for a futa MC, I ended up having to go into my save file with an editor and change my MC gender to male to trigger the flag allowing me to select the option and then changed it back afterwards. I am not 100% myself on this since I don't entirely know coding, but I think the futa gender might be flagged as female at some point in the code as I have noticed that during sex with women with out the bisexual trait they will complain about the character being female, but males without the trait won't make any mention of it themselves.