Sexual Disgust:

Microbes, Mating, and Morality - Joshua M. Tybur et al.:

"we suggest that sexual disgust is an evolved solution to the adaptive problem of avoiding biologically costly mates and sexual behaviors (see also Fessler & Navarrete, 2003).

Over evolutionary history, one’s choice of sexual partners and behaviors carried significant reproductive consequences because individuals varied in qualities impacting offspring survival and reproduction. Whereas certain sexual partners increased the probability of producing multiple, healthy offspring, others potentially jeopardized one’s reproductive success. Given this selection pressure, natural selection likely favored mechanisms that were able to evaluate potential partners along dimensions relevant to reproductive success, and systems that motivated pursuit (e.g., lust) or avoidance (e.g., disgust) accordingly.

Importantly, potential sexual partners can vary in quality along two broad dimensions: intrinsic quality and genetic compatibility (Jennions & Petrie, 2000; Neff & Pitcher, 2005). A mate’s intrinsic quality is reflected in features that influence objective physical attractiveness, regardless of genetic compatibility. Such features include body symmetry, facial attractiveness, and body shape (e.g., Grammer, Fink, Moller, & Thornhill, 2003; Singh, 1993; Thornhill & Gangestad, 1993, 2006) – dimensions that males and females use to assess attractiveness.

Genetic compatibility affects mate suitability in a more relative manner. A potential partner’s genetic similarity to oneself – rather than intrinsically low genetic quality – can reduce reproductive success. Compatibility can be influenced by factors such as major histocompatibility complex similarity (Penn & Potts, 1999) and genetic relatedness. For instance, though one’s close kin (e.g., siblings, parents, offspring) might possess many attributes desirable in a mate (i.e., have high intrinsic mate quality), they are not suitable mating partners because close inbreeding increases the probability of producing less healthy offspring (Bittles & Neel, 1994; Charleswoth & Charlesworth, 1999; Haig, 1999).

Individuals displaying cues for low intrinsic quality or low compatibility are likely to be poor mate choices, and should thus be avoided as sexual partners. Disgust is an emotion wellsuited to perform this function. The disgust that motivates sexual avoidance, however, is distinct from the disgust motivating pathogen avoidance, not only with respect to the sets of information required to assess mate suitability versus parasite presence, but also in regards to the nature of the optimal avoidance behaviors. Whereas pathogen detection relies on cues such as puss and foul odor, the assessment of mate suitability depends on a host of other cues described above – many of which are not relevant to proximal pathogen avoidance (e.g., seeing one’s mother care for a newborn, a cue to siblingship; Lieberman et al., 2007). Further, whereas individuals and objects displaying cues for communicable infection should motivate general avoidance, an individual deemed an unsuitable mating partner should motivate avoidance specifically within the context of mating, leaving open the possibility for other categories of social interactions (e.g., nepotism, friendship, social exchange, or group membership).
 

In sum, avoiding sexual partners and behaviors potentially jeopardizing one’s reproductive success constitutes a separate adaptive problem from pathogen avoidance and requires different systems for assessing the risks associated with sex. Sexual disgust, we argue, is specifically well suited to perform the function of avoiding reproductively costly sexual behaviors, narrowing the pool of sexual behaviors and partners to those likely to contribute to the production of healthy viable offspring."