 Consequently the kind of wrongful sexual choice most often considered by Aquinas is engaging in intercourse with one’s spouse without fides, because one either (i) is thinking of one’s spouse in the way one would think of a prostitute, or (significantly worse) (ii) would be willing to have sex with somebody else if some other attractive person were to be available. Such depersonalized sex acts are instances of willing against the good of marriage (contra bonum matrimonii). This reiterated analysis should be regarded as the key to Aquinas’ sex ethics. Another paradigmatic kind of instance, in itself much more serious, is a married person’s choice to have intercourse with some third party (perhaps with the other spouse’s consent). These and all other wrongful kinds of sex act have their wrongfulness, according to Aquinas, not because they are unnatural in some biological or sociological sense of “unnatural”, but because they are against reason’s directive to respect, if not also pursue, the good of marriage, a respect that calls for reserving to marriage (and within marriage to truly marital intercourse) all uses of one’s capacity to engage in the intentional pursuit of sexual satisfaction. For unless one regards such reservation as required, one’s stance about human sex acts is contra bonum matrimonii and unreasonable because one cannot coherently maintain that the intercourse of the married enables them to actualize and experience their fides and their marriage, a thought essential to the flourishing (bonum) of marriage and thus of children and thus of the wider community as a whole. One measure of the gravity of morally bad sex acts is applicable to some kinds but not others: injustice (as in rape or seduction of the vulnerable). Another measure, applicable to all such acts, is the extent of the deviance (“distance”) between acts of that kind and truly marital acts.