Can I write romance works despite never being in a relationship?
The reason those sorts of mistakes happen and people unfamiliar with a particular work or haven't researched it enough get exposed is that the original work holds all of the right answers.
Soccer has rules too, so when you come across a scene in which a character runs toward the goal with the ball under their arm, it becomes apparent that the author knows nothing about soccer.
On the other hand, there is probably no other world like the world of romance, which has no rules or "right answers".
As many unique relationship configurations as there are, there are as many different ways to be in a relationship. Nowadays, a growing number of people in the sex industry have gone beyond penetration and ejaculation and leave it to the parties involved to define what constitutes sex.
These new interpretations of sex are our forte, or rather, maybe we're catching up with reality. "Okay, the seme and the uke appear in the same panel. Does this count as sex?"
Some forms of love may be beyond our understanding, but that doesn't mean they can't exist.
Therefore, we shouldn't be too concerned when people read our love stories and say, "This doesn't make sense, the author must have zero romantic experience." In fact, those are just insensitive remarks from someone who can't see beyond their nose, who believes their common sense is everything, and who hasn't kept up with the diversification of the world.
Even if you have no romantic experience, the most intense form of love you can imagine is most likely a right answer in itself.
You don't need experience, just knowledge
In other words, with the proper knowledge, you can realistically portray things, even with no experience.
Very few professionals draw entirely from their own experience and imagination, while many draw by "studying," learning from materials, or watching other manga and movies for inspiration.
You don't need to convince anyone else
The fuel of fan work is the DIY spirit of creation, the idea that "if there isn't a single story in the world like the one I'd like to see of my OTP, I'll make one myself."
You see, there's no need to convince others. There never was.
The essence of fan work isn't to create a love story that others will approve of, but to create a love story that you want for the characters.
Even if it's sex that involves sucking on unopened condoms, if your reasoning behind it is "I wanted to see my OTP do something mischievous like that," even pointing out that the characters are using condoms incorrectly would be impolite.
It's only natural to be concerned about the eyes of your readers when you publish your work for others to see, but the most important reader of your fan work is always yourself.
First of all, depict what you want to see. Then, if someone comes up to you and compliments you because those two characters doing it while sucking on a condom was exactly what they wanted to see too, then lucky you.