They are really erogenous zone when done right. It really gives you shiver through your body when properly stimulated.
Also, Jay is doing that because the first time he blew Shingun in that back alley, he teased him there when he was fondling him and realized Shin was really reacting to it. More than the BJ in itself.
So it is not the Jay has a fetish. It is more that he wants to pleasure Shin completely with what he knows knocks Shin’s socks off.
So is shingun the one with a fetish?
It is not a fetish. That is not what a fetish is.
A fetish would be the idolization of a singular non-sexual thing (foot, hand, golden shower, breast feeding, wearing diapers among others) onto which you transfer all your sexual desire, attraction and gratification, resulting in an over-sexualizing of it. Shingun does not center his sex activity around armpit licking. He does not idolize his own armpit, or create a power play toward it.
He just reacts to it the way one could react to French kissing, cunnilingus, blowjob, nipple play, rimming, neck bite or a simple massage or caress. It is a form of foreplay. For example, even if foot fetish involve foot action, licking toes or getting your toes licked during sex does not necessary mean you have a fetish, if you do it just for the feeling and not because you have a sort of venerating ritual toward it. It just means you like it and know what your body or your partner's body react to physically.
It could be qualified as a kink, but not a fetish. Shingun reacts to it, probably because he has an erogenous era there (there is a lot of nerve in that region after all) and Jay does it because when you love someone and want to be intimate with them you gonna want to do the thing that give them the best tingles.
And even though Shingun grabs Jay's breast and ass, because that turns him on to feel firm bubbles under his hand, it isn't yet at the level of a fetish. It is still at the level of a kink. Also Jay seems to react to it as well, and that is something we know from their first sexual encounter when he definitely reach orgasm faster after Shingun started to fondle his rear. Which prompt Shin to do it more. But it doesn't seems like Shingun is attracted to any firm chest and ass he sees. It is attracted to them because they belong to Jay and he finds him sexy. Not the other way around. Which is the key element of a fetish.
In a fetish case, the partner is chosen as a mean to satisfy ones sexual focus, while in Jay and Shingun's cases, their sexual focus are done as mean to satisfy their partner.
That is why Jay licking armpit and Shin grabbing ass aren't fetishes. They just understand each other body really well and both are really into each other, in a very desperate, passionate and brutal way.
I don't really understand the whole armpit and ear thing but I'm honestly not complaining