OED has a very early citation:
c1394 P. Pt. Crede 375 Þey ben digne as dich water þat dogges in bayteþ.
In this case, digne doesn't mean dull, it's related to dignity and OED has it defined as "Having a great opinion of one's own worth; proud, haughty, disdainful; (cf. ‘stinking with pride’)".
Ditchwater is generally muddy and not clear: it's dull. And it can be smelly. The translation appears to be "They are as smelly" [or "Their pride stinks"] "as ditchwater that dogs have drunk from."
Google Books has "dull as dishwater" appearing in The Amaranth published in Boston in 1854.
First my "coincidence" answer
Animal metaphors for people are common. Cats are a familiar species and likely to be the source of many and varied metaphors (see also farm animals and dogs, but note that cats are more independent than either and thus more suitable as a metaphor for a person with agency). It's quite likely that (in a strongly gendered society) there will be a bias towards references to one gender; this bias will tend to be reinforced by usage).
Now the (not mutually exclusive) answer based on sexism
Cats, in contrast to dogs, are seen as cunning, employing trickery rather than good honest brute force, as well as sleeping a lot. Their sounds are also either high-pitched or gently soothing (though purring can be a sign of fear), while dogs make more aggressive noises. Cats are also prone to appearing as if by magic and to staring at things people can't see (a little fly in front of their nose but the other side of the room from their human) as if watching fairies. These all fit with human gender stereotypes, where clever women were seen as manipulative and/or witches, and women who worked hard behind closed doors seen as lazy daydreamers.
Thus many of the traits associated with women are similar to those associated with cats. While foxes display some of the same traits, they're less familiar and when confronted with an angry human male will flee while a (fully domesticated) cat will purr and stick around.
This is of course speculative; to back this up with usage would be tricky as we're going back a long time in a case where oral patterns matter.
Some history
Etymonline dates "catty" to 1886 without gender as "devious and spiteful" (ngrams shows plenty of earlier use).
Oxforddictionaries.com has "she-cat" as (¹a female cat or) ²a spiteful woman, with the origin given as "Late 16th century; earliest use found in Thomas Lupton (fl. 1572–1584), political and religious controversialist." It's not completely clear that this first use applies to the second meaning.
Shakespeare can normally be called upon for colourful descriptions of both men and women, but despite his "using them as metaphorical tools to express the opinions of women and to describe women’s nature" (basically a blog) the uses I can find aren't general about women; some are about men: opensourceshakespeare.org search results for "cat".
A few centuries later, Dickens appeared to like his cats (in fact he had his own cat stuffed after it died) but his uses seem to have been alomst all literal; the few figurative uses I've are as likely to apply to men as women.
This has been studied
If you have journal access, the cat as a metaphor has been studied:
- comparing English and Malay with no mention of gender (but the bibliography looks helpful).
- A study of English and Spanish use of anmial metaphors for women. The section starting
3.1. Women as pets
Within the animal world, pets enjoy a privileged position. In
fact, pets are not conceived of as beasts of burden nor are
they killed for their meat or skin. On the contrary, pets share
the same roof as their owners and their main function is to keep
people company. This benevolent attitude towards pets finds its way
into the English language, for the very word pet is used as
a term of endearment
is likely to be of particular interest (some woman-cat metaphors have Spanish counterparts).
Best Answer
The answer to this may well be dependent on the local culture.
In the UK, I certainly would not assume that a host is male.
Moreover, if using the word hostess, I would perhaps be careful of context for the reasons apparent from the definitions in Chambers Dictionary:
Thus, some female hosts may well prefer not to be referred to as a hostess.
Additionally, the first definition given for host is:
Thus confirming that host is an appropriate term irrespective of gender.