I'm a little curious about this, but what could Slattern's actual gender be? In the trivia section of the Slattern article it says:
"Slattern" is a term that dates back to the seventeenth century. It refers to a sexually promiscuous or dirty and untidy woman."
Now, yes, the article also mentioned that Stacker Pentacost also referred to Slattern as "Big Bitch" in the novelization, but usually the term "bitch" is used as an insult, regardless of one's gender. Personally I think it's because Slattern walks on all fours.
Additionally Slattern could be female, being the largest kaiju encountered. It's quite common in amphibians, reptiles and insects where the female is generally larger than male counterparts, but could this actually be true? Please do fill me in with this; i'm a little uncertain.
We don't know. The answer simply isn't out there. The only reason we even refer to Otachi as female is because she gave birth, which usually (but not always, as hermaphroditic creatures and seahorses will tell you) means femininity.
Actually, in seahorses, the female actually lays the eggs, the male incubates them in a special pouch on the abdomen. It is sort of like male penguins holding the egg until it hatches, but the female actually lays it.
The name likely provides no information for the gender, as the names are usually meant to be insulting, rather than descriptive(with knifehead being an exception). Also, since the kaiju are artificialy engineered, the size would not matter.