The Disgust Response in Human Mating:

"Sexual attraction and arousal fulfill important functions in mating decisions: They motivate courtship, copulation, and pair bonding with individuals of high sexual value. The absence of attraction and arousal could thus potentially perform the function of steering individuals away from mates of low sexual value. However, the absence of sexual arousal would not prevent that individual from being sexually pursued by other people who possess their own reproductive agendas. To reject and avoid unwanted sexual advances and behaviors another response is required. Emotions such as fear and anger are not well suited to avoiding potentially costly mates. Fear can lead to immobilization or rapid flight (Öhman & Mineka, 2001) the former of which likely does not impede sexual pursuit, and the latter of which is metabolically costly and often unnecessary (e.g., if social allies and kin can prevent another’s sexual interest from progressing to sexual aggression). Similarly, anger often acts as an “approach” emotion (Carver & HarmonJones, 2009), and associated aggression can lead to costly counteraggression (Sell et al., 2009). 

We suggest that the phylogenetically ancient (Curtis, 2007; Zhang et al., 2005) pathogen disgust was a felicitous system to co-opt to perform the function of avoiding biologically costly mates."

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"The lack of a constant state of sexual disgust toward poor mates reflects the costs associated with avoiding individuals who are otherwise valuable social partners. Although constant motivations to avoid poor mates would certainly decrease the probability of reproducing with them, it would also cripple some beneficial social relationships."