@MxVerda - haha you'll get a few different definitions for that, because both "sex" and "kink" are things that different people define in different ways!
Broadly, "kinky sex" is when you're doing both sexual activity and kink activity at the same time.
Some people make kink a huge part of their relationship and include it in a lot of different aspects of their time together. For them, all their sex is probably kinky sex because they are always "sceneing" together, to some degree. I personally happen to find that amount of roleplay exhausting! But for some people it's exciting to be "in character" at as many times as possible. For those people there would be an obvious difference between when they're just doing "kink" and when they're doing "kinky sex", though, because when they're (for example) doing the grocery shopping while including subtle kinky play (using titles for each other, making displays of service like opening doors or waiting for them to speak first, wearing hidden jewelry or underwear beneath their clothing, etc), well that's just kink not sex.
Other people only engage in kink play occasionally, so they might have some sex that's clearly "vanilla" (non-kinky) in nature and other times when they're dressing up, pulling out the riding crop, etc.
There's a whole spectrum between those things, obviously!
Edited to add: culture helps define what is kink in a given context. Oral, to take your example, was only kinky at a time and place when it was culturally taboo.
Mx Verda
Ryan Clough 
M.S. Bellows, Jr.
Erin Ptah
Polywog 🏳️🌈 (she/her)