Meaningless sex, in the parlance of females, is sex that they have with a man for free, rather than in exchange for betabuxx. When they get caught cheating, they assure their boyfriend or husband that it was just "meaningless sex" and therefore not a big deal. The implication is that there was no emotional attachment with the person they were cheating with, when in reality it is their betabuxx whom they are using in a mercenary way, because he does not excite and sexually arouse them the way their illicit lover does.
The idea of "meaningless sex" is illustrative of the opposite approach that men and females take toward relationships, in which females seek to use sex to get men emotionally attached to them, and men seek to elicit female emotions that will make them want to have sex. From a females' perspective, if her man has sex with another girl but doesn't get emotionally attached to her, then what happened was not as bad because at least the other girl did not capture his heart and thereby take the place of his main female. From the male perspective, if a man's wife gets emotionally attached to another man but doesn't have sex with him, then the process of seduction was not completed, because she was not physically taken by him.
Men tend to be more concerned with the sexual aspect of ownership of females since they do not typically rely on romantic partners to the extent foids do for leadership, protection, provisioning, emotional support, etc. Men also have more need to assure themselves of the exclusivity of their foids' sexual devotion to them because of the instinctive aversion to being cuckolded.