its the seeds that have gone rotten...
it sometimes happens, but rare.
sometimes you get apples with brown areas of mush inside,, or avocados with black areas of mush inside,, but you just cut it out and eat the rest.. (or discard it). but it does not put you off eating apples and avocados.
its just a rare defect. nothing is perfect ;)
so there is no need to be put off bananas. Give them a chance to have the occasional defect :)