Hey, sometimes I have short rants! (No need to use ten paragraphs to say something when two will do!)
* There's a personality problem troubling your game? All too often, people kvetching to gaming forums about them want the readers to tell them how to solve them without actually having to open their mouths. In a hobby where most of a GM's job, for several hours in a row, is
communicating with the players, I'm constantly flabbergasted at how many
of them claim that they have trouble doing so. This magical thinking -- that there's some way to make evildoers just Get Better without a word being spoken -- is all too common.
There is no way, none at all, to change your players' behavior other than to have an open, adult conversation about your concerns. Any other way of "nudging" people in one direction or another does not work. Never has worked. Never will work. The clueless don't notice, the jerks don't care, and the ones waiting for the aforementioned open, adult conversation resent what they see as clumsy manipulation attempts.
* On gender and same-sex relationships: It is not my bloody job to dictate to any player the gender and sexual preference of a character. It is my job to provide – and portray – the NPCs with whom the PCs interact. These will be male or female, straight or gay, romantically interested or not, as circumstances dictate. My masculinity is unthreatened when I play a gay NPC. Or a female NPC having a relationship with a male PC. Or a female NPC having a relationship with a female PC. Whatever. Because I'm not six years old any more, and I see no reason for my reflexive 1960s prejudices to affect my grown-up life.
Screw the squick factor. If a PC wants to murder someone else, do I go all squeamish on him and tell him he can't do it? If he wants to torture someone else, do I go all squeamish on him and tell him he can't do it? Beatings, theft, torture, racism, genocide, slavery, sacrilege, arson, drug use, maiming, murder retail or wholesale, I can set the table for all of it. Torch a village, desecrate a temple, debauch virgins, kick puppies, slit throats, most of us are cool with all of that, but almost uniquely, tabletop gamers draw the line on portraying male-on-male romance?
* “You’re/He’s ruining my fun.” I stay far, far away from that turn of phrase, if I can possibly help it. For one thing, "You're ruining my fun" far too often is a code phrase for "I'm a self-absorbed solipsist, and I take failure to conform to my prejudices and whims as a personal attack." It's hauled out as a trump card perceived to end all debate, without examination of how that behavior actually might be "ruining" the speaker's fun, or whether the speaker's POV is reasonable. I see no reason why it should be used as an excuse to dictate to players what otherwise-legitimate character creation and play choices they’re permitted to pick.