Like people have said before me, romantic sex is pretty subjective...
To me though, I consider it when there's that really intense connection between my partner and I; when you hit that pure bliss/transcendental state together
For each partner I've had, this ended up being different though!
Like for example, my current girlfriend enjoys things slow/gentle and can't physically handle overly rough sex. Even though that may not be my preferred type of sex; me knowing that she's enjoying it, then she sees that I'm making the effort to make sure that she's enjoying, usually brings us to that connection and makes it what I consider to be romantic.
On the other hand, with other partners where the sex was very rough, lustful/aggressive, kinky, etc, we've still been able to reach this state. (I can also still have fantastic, detached sex even when it's work related, but I don't find it romantic unless it has the emotional connection in it) So, in my opinion, it definitely depends on the relationship/dynamic and what you collectively find romantic!
(Keep in mind though that I'm a Pisces and ENFP, so take this with a grain of salt haha)