In my guppy tank i have definatly noticed that with the females and males(2males 6females), some females seem to prefer mating and hanging around with particular males in the tank. I used to have a blue male guppy which none of the females liked while with my red patterned guppy the yellow and grey females seem to prefer him more while with my red and blue patterned male guppy, the tuxedo and one of the grey guppy females prefer him.
I make it so my fish can't interbreed with each other but there is one exception; i have two platys in particular which are brother and sister and although they are very close to each other(they are always together), the female always mates with the other males which are not related to her and has never produced fry belonging to her brother.
Also, a while back i used to have a realy violent male platy who picked on all the females and the brother platy of the two always defended his sister platy from the male even though it was not because he wanted to mate with her. So maybe it is posible for platys to have relationships with each other that are not related sexually but by family...