again, I'm no scholar on these things, but it's not just a poor story, it's the amount of story. If the plot:sex ratio is too much on the sex side, it's either erotica or porn.
The other part of it is the question, what is served by the sex scene? if it furthers the emotional arc of the characters, it's either erotica or romance. If it's just there for the sake of having a sex scene, it verges into porn (again depending on the amount of plot and the graphicness of the description of sex)
in romance, it's not just "sex" it's "making love" because it's the emotions that are important.