Fandom Considerations
Anne McCaffrey has stated in a number of documents and interviews that dragonets use pheromones to determine the sexual orientation of the humans to whom they Impress. According to these statements, greens Impress only to women or to "effeminate" homosexual men. Blues Impress primarily to homosexual or bisexual men with "masculine" temperaments, or possibly to masculine or lesbian women; browns similarly Impress primarily to heterosexual men, but sometimes to bisexual men. Bronzes and golds Impress exclusively to heterosexual men and heterosexual women, respectively.
However, these ideas have never been made explicit in the books (although it is clear, at least, that most male green- and blueriders are homosexual). Many members of online Pern fandom find McCaffrey's ideas about sexuality highly questionable for a number of reasons, both scientific and ethical. (Most infamously, she claimed in an interview that science has proven that being the receptive partner in anal sex triggers a hormonal change that will make a previously heterosexual man become homosexual and effeminate. Thus, she argues, even if a male greenrider were originally heterosexual, he would not stay that way.) In later interviews McCaffrey claims that green dragons merely pick up on psychological clues from homosexual boys before they themselves know that they are homosexual. "A green Hatchling is unlikely to be impressed (pun intended) by a heterosexual boy." - Anne McCaffrey 1998 on The Kitchen Table BB.
Pern-based roleplaying games thus sometimes ignore McCaffrey's restrictions on who can Impress to a given color of dragon. MUDs and fanzine-based clubs often ignore everything except the basic rule that only women Impress gold and only men Impress bronze; PBEM games are more likely to accept the restrictions on sexual orientation. Most clubs post their policy on canon strictness. While some accept more liberal thoughts on color/gender/sexual orientation matches, many are very strict on this issue.
For the purposes of roleplaying games, McCaffrey has also officially allowed females (masculine lesbians) to ride browns or blues, though she insists that this could never happen on her (canon) Pern.
Also in fandom, if a rider has strong objections to sex with someone involved in a mating flight or the writer has objections to writing a homosexual encounter or object to their character being involved in a sexual encounter with a person other than their "significant other," they may sequester themselves with a more acceptable partner during the flight. This idea is called "Stand-Ins" and based on a concept McCaffrey introduced in Dragonseye/Red Star Rising, in which a female greenrider objects to the idea of a specific bronzerider winning her green's mating flight. However, this concept is not seen in other books and clearly does not exist in the 9th Pass, as several problems arise regarding green and gold riders who object to the random nature of mating flights and end up raped by the male winner of the flight (most notably in Skies of Pern).
Read more about this topic: Dragons (Pern)