The most resilient gender conventions in roleplaying games?

[Cealocanth]

The evil dragon that must be slain can be either gender, but if it is a gold hoarder and princess capturer it is pretty much always male.

Troglodytes, kobolds, goblins, gnolls, and pretty much any other bestial monster-fodder race is pretty much universally male. (Does our society think that it is more ethical to kill males than females? This is probably why you never see female mooks in modern day spy movies either.)

The wizened wizard in the party is almost never a wizened witch.

The elven archer is usually female, despite the genre convention being a male character.

The dumb fighter is always male. If there is a female fighter, regardless of her Intelligence stat, she will not be the dumb fighter that hits things until they break.

The mad scientist is always male. (Not exactly fantasy, but still interesting)

The mob boss, crafty merchant, or politician character who attempts to usurp power using coin and guile is always male.

The Dwarf is always male. Always. Even if a player is playing a female dwarf, they will always be of the druidic, priestly, spellcaster, or tinker varieties. The beer-guzzling axe-weilding honor-obsessed Tolkien Dwarf is always male.

The Gunslinger is usually male.

The Lord of All Hells is never, never, The Lady of All Hells. This probably stems from the biblical version being male, but still you rarely see people try to subvert this one.

Drow are always matriarchal and you are always hard-pressed to find a male drow.

Pretty much every Fae race is restricted to one gender or the other. Fairies and pixies are female. Fauns are male. Mermaids, Sirens, Dryads, and similar spirits are female. The Ent are usually male. Funny how you don't see this one subverted very much, with the exception of Gnomes, which are actually remarkably equally played across gender lines.

The Werewolf is always male.

The female PC with the highest charisma is almost always volunteered to seduce the guard, regardless of sexual orientation or willingness to do so. Assuming there are multiple girls at the table. Otherwise the only one gets volunteered.

That's a funny one. It's weird, actually, that when you find the need for an escort, every player at that table will pretty much instinctively point to that particular female player, even if an equally charismatic male character is available. I think there's probably some underlying thing in our society which is more willing to accept females as seductresses then males as seducers. This is probably why succubi tend to outnumber incubi 10 to 1.