I was getting ready for my daughter's PTA meetings and I was trying to figure out what to wear. I was thinking it would be funny to wear a women's bikini, but my ex-wife said I would be jeopardizing our child's future. I said that was horsecrap and threw a temper tantrum and I came THIS CLOSE to choking a bitch. Then my grandmother walked by and said "You can't have your cake and eat it too!" I stopped dead in my tracks and had to sit down and think about what this elderly woman just said to me. I can't have my cake and eat it too? Yes I can! I can have a cake and then I can eat it. I wouldn't just stare at a cake, that's insane.
The only reason to buy a cake is to eat it and in the time between purchasing it and eating it you're "having" it. So yes you can have and eat a cake. I ran up to my grandmother's bedroom to make fun of her and tell her how what she said made no sense and she explained that it means "You can't have it both ways." She meant you can't have a cake in your possession AND eat it because when you eat it, the cake is gone so you no longer "have" it. I told her if that's what you mean then just say "You should never eat a cake if you want to keep it." But even that's stupid because there's no point in just keeping a cake. Everyone ends up eating the cake in the end otherwise you have to throw it out.
If you're trying to illustration a choice between two equally important things then this saying is stupid because possessing a cake and eating it are not comparable at all. In the end I decided to wear the bikini to the PTA meeting. Everyone laughed and my daughter's teacher saw that I had a sense of humor and we seriously discussed her grades and what to do about tutoring and such. It turned out great. I had my cake and I ate it too. I ate a cake and I still have it, because it's still in my stomach. I am possessing the cake in my stomach even after I ate it. That's a dumb saying.