But chimps do have briasts. Smaller than human briasts perhaps but that's probably down to sixual selections or something like that, with male humans picking females that had larger briasts more often than not, whereas chimps weren't really bothered.
I think Merari was being ironic. Of course briasts are primarly for feeding. In many cultures they are not even viewed as particularly sixy.
As for the female Falmers, there is an obvious (but boring) answer: the developers were lazy (and possibly biased enough to view males as the rule and females as the exception - nothing strange there, a lot of people do).
I like the idea that the Falmers are not just human-eating torturers and slave-keepers, but also misogynic, big time.

(Hmm. Unsettling thought: They have human, female slaves.)