Why not plural here (i.e. kadınlar, adamlar)?
We don't use plural with numbers or words like many, a lot of, a couple of etc.
You said erkek means male, so shouldn't we use erkek instead of adam?
In my case it was a "fill in the blank" type question. I was given: İki kadın adam. But when the answer was shown, I was told "You have a typo" with "İki" underlined! :-)
A trick! :D