Most popular fetish?
Who knows for sure? Feet, footwear (especially high boots), leather,diapers, PVC and latex are all common fetishes. Judging from the questions on this site, various forms of foot fetish are far ahead.
Foot fetishism is one of the most common fetishes.
One fetish that I love is wrestling. I like to wrestle with whomever is my current girlfriend and sometimes they think it is weird but to me it is a form of stimulation without penetration, although sometimes she will get worked up enough and penetration just becomes part of the equation.
From the standpoint of human reproduction, yes -- it gives people an additional reason to find mates. Ethically, a fetish in and of itself is neither "good" nor "bad". What counts is behavior. If the fetish causes someone to misbehave (to force someone to have sex, for example) then it is obsessive and dangerous to others. The most straightforward advice: you should control the fetish. It should not control you.
A "fetish" is a sexual response to something that is not usually considered sexual in and of itself, like red vinyl, cigarette smoke, boots, etc. Fetish sex is not "normal" sex. It is a sexual act geared around what most stimulates the participants whether it be bondage, PVC, being dipped in banana pudding or any number of things.
You will find that most do not enjoy this experience at all. It is apparent that you have yet to experience it. Oh, wow. Who doesn't enjoy a swift kick to the cherries? Are you NUTS? There are over 10,000 nerves in the scrotal area. It freaking KILLS. It's called a ballbusting fetish. They are sites dedicated to women kicking men in the balls just look it up. I don't know how the fetish arose…