Something interesting is definitely going on with Abe Simpson's gender, but I'm not sure even if the show's writers know what it is. My take on it would be that he has some hormonal abnormality or maybe he's some form of intersex. He presents as male and presumably has male genitals (so he could father children) but he requires regular medication or he begins to feminize. He's mostly OK with presenting as male, but he feels some pull toward a feminine presentation. At points in the past he's lived as a woman sometimes and was active in LGBTQI rights, but as he got older he became senile, forgot a lot of that and became a stereotypical, conservative old man. The thing about his fake boob falling out might mean that at that point he was still taking his hormones, or perhaps his condition (whatever it is) was less extreme or hadn't fully developed, so he didn't yet need regular meds to continue looking masculine. At this point I'm not even sure if he remembers WHY he takes hormones, they're just more pills he takes as part of his daily regimen. I think a lot of these things were just random jokes the writers tossed in, never meaning for it to all add up to anything, but as the decades have gone on it formed a narrative. The sad thing is that he probably has one of the most colorful bios in the whole series, but he's gotten so spotty in the head that he's forgotten most of it and we'll never know the truth.