"Male homosexual behaviour can be found in most extant classes across the animal kingdom, but represents a Darwinian puzzle as same-sex mating should decrease male reproductive fitness," they wrote.
However, many animals that engage in homosexual behaviour have also been observed mating with the opposite sex, including penguins and bonobos.
Biologists have suggested that such actions could still reap genetic rewards, despite the perceived lack of reproduction, through knock-on effects.
Atlantic molly males "nip" near the genital openings of potential mates to signal their readiness to mate.
Scientists have suggested that such behaviour helps to demonstrate the quality of males, because their level of exertions can indicate overall health and virility.
Yet subordinate males are known to nip both females and other males.
Studies of the fish, which are found from Mexico to Guatemala, have shown that they can discern the sexes based on pheromones and visual cues, undermining any theories of misrecognition.
Females choose their mates in molly society
So, in order to understand the motivations behind this behaviour, the German scientists studied Atlantic mollies in their lab.
Using animated recordings, the team tested how "attractive" the fish found different examples.
While the females found a colourful male more attractive than a drab counterpart when swimming side by side, they reacted better to "less attractive" males once they had observed them nipping either males or females.
"We were quite surprised to find out that observed homosexual interactions had the same influence on females' preferences as heterosexual interactions," explained Dr Bierbach.
"The implications are that sexual activity per se is a trait used by females to evaluate males' quality and that our results could also be true in other species."
The scientists speculate that it could be a tactic employed by smaller, subordinate - and thus less attractive males - to win more female attention.
"Males can increase their attractiveness towards females by homosexual interactions, which in turn increase the likelihood of a male's future heterosexual interactions," Dr Bierbach told BBC Nature.
"We do not know how widespread female mate choice copying is, but up to now it is reported in many species, including fruit flies, fishes, birds and mammals [including] humans."