OMGrey analyses the strength of Inara Serra:
Inara is a companion. In the world of Firefly, this is similar to a Geisha or a Courtesan, a high-class, well-educated sex worker. Although Mal seems to take pleasure in calling her “whore” and other such misogynistic attacks on her choice of career, as the audience gets to know the characters it becomes clear that he says these things in order to remind himself and attempt to keep an emotional distance from her, because he is deeply in love with her. And she is deeply in love with him. The tension between the two is undeniable. Then the question remains: why don’t they ever get together?
The easy answer, of course, is that Joss Whedon is a sadist, which he is when it comes to romantic relationships in his stories, or that the sexual tension is necessary for plot or conflict purpose. Certainly the above two reasons are true, but the more important answer lies within the characters themselves. In fact, it is what helps define those characters.
Love is powerful. Sexual desire is powerful. Combined, they are virtually impossible to deny, especially when one is in such close proximity with one’s beloved day after day after day. Additionally, Inara and Mal are normally in the middle of space for weeks on end without seeing anyone other than the other members of Serenity’s crew.
Only someone with amazing strength could deny such a strong attraction and deep love. Day, after day, after day.