For my own amusement, and because it's what we're doing in Philosophy class right now, I'm going to analyze the issue using Kantian ethics.

1. The assumption that being nice should lead to sex or relationships goes against the categorical imperative: humans are an ends, not a means, and their dignity as such should be respected. When saying being nice should lead to rewards, one disrespects oneself by seeing personality as a means to an end. One also disrespects others by ignoring their human dignity and seeing their existence as a mean to an ends (in this case, sexual gratification).

1.1 Another wording of the categorical imperative is that our maxims should only be those maxims which work perfectly well if they were the categorical imperative (i.e. universal). On the face, this may look like the Golden Rule, but the Golden Rule is consequence based. Saying that the assumption is wrong because then almost everyone would thindk they deserve sex (most people seem to think they are nice), which would lead to the exact same issue as currently (i.e. people who should not be in relationships, like Henry, are, but now it's even morally right) takes the same consequence-based approach. Kant might say it is wrong because applying that maxim across the board would not be rationally motivated but motivated by desire only, and therefore corrupts the rational categorical imperative.

(Note: Kant thought the two wordings of the categorical imperative were equivalent. No one else can really figure out why. Maybe one of you can. I've got sown ideas, but they're just barely budding.)

2. However, that doesn't necessarily make the assumption, or actions based on the assumption immoral. When we are acting on goals and desires, we are acting solely as animals, not rational beings. As we cannot choose our goals, we cannot truly be held responsible for actions towards them. It is generally agreed now that sexual preference is not a chosen trait, and so having the goal of sex is not a choice. Just like cheetahs cannot be blamed for eating the gazelle or a ball can be blamed for falling, humans cannot be blamed for trying.

3. At the same time, looking closer at the issue suggests maybe it isn't all about desires. While humans cannot be held morally at fault for simply pursuing goals, means and motive matter for Kant. A person has an obligation to use their rationality to approach their actions. Also, as rational beings, we have a duty to follow the categorical imperative, only for the sake of that duty, i.e. doing right. And rather than just pursuing sex through being nice, the assumption of getting sex from being nice shows an attempt at rationality. In effect, it is a strange and incorrect attempt to usurp the true categorical imperative arrived at by Kant. And rational choices are morally judgeable. So, Kantian ethics would (likely) say the assumption and any actions resulting from the assumption are wrong. Of course, Kant said all sex outside of marriage was wrong, so the issue has a simpler answer. But Kant isn't modern, and almost certainly never thought about nice people thinking being nice was enough to be in a relationship.