Niche vs Kink

I honestly don't understand the concept of these two when it comes to erotica especially on smashwords.

For example: Is gay erotica a niche and can I explore all type of kinks through this same niche? Or do readers like when you stick to only one niche and kink?

And if I'm writing straight erotica, can I also explore all type of kinks or will that be disregarded as being too vast in straight erotica?

Is BDSM a niche or a kink? Is missionary a niche or a kink?

Is stepfamily stories a niche or a kink? Is inc*st a niche or a kink? I'm confused by all these and I would really appreciate if someone clarified this.

Another thing: Is it true that on smashwords general erotica does not perform well compared to taboo erotica and general romance books?