From Nature:
It might sound unlikely, but men looking at explicit pictures of two naked men with a naked woman have been shown to produce higher-quality sperm than those watching pornographic images featuring women only. Although this seems to go against common perceptions about male sexual preferences, it is consistent with the theory of sperm competition, says study leader Leigh Simmons of the University of Western Australia, Perth. This states that males (of many species, including humans) should produce better sperm when faced with a female who has other mates, because this stimulates them to boost their chance of procreation.
The findings may help fertility clinics to obtain the best possible sperm samples from their clients, by providing specialized images of intercourse for men to view. This might help prospective fathers maximize their fertility, Simmons suggests. Though he adds that some women may disapprove of their partner viewing such material.
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