I mentioned a couple of days ago that I'd made a video with Herman - a useful one, for a change. :-) Here it is. It's about how you can tell the sexes apart in turtles of the Chinemys species. It covers pretty much all the points you need to tell the sex of your Chinemys turtle. Only thing missing is that I only have males to show and no female. But there are some crucial features that the latter will be lacking that should mean you'll easily be able to tell anyway.
So does it matter which sex a captive freshwater turtle is? Yes, definitely, and for three reasons. One applies to all of them, and the others to ... not all species, but a lot. The last two reasons first: Size. A lot of freshwater turtles have significant sexual dimorphism - that is, physical differences between the sexes. Since these animals are oviparous, the females tend to be larger. And in Chinemys species, this size difference can be significant. As I mention in the video, any Chinemys larger than about 6" shell length will probably be female. But the females can also get a lot bigger than that. Worst case scenario the female will be twice the size of the male or more. o_O This has an impact on the enclosure you need to provide for your turtle - of course all captive turtles should be given as much space as possible, but a female Chinemys will require more space than a male.
If you have more than one specimen, the sex distribution is also important. Say one female and three or four males - that will usually be very unpleasant for the female, as she will receive a tremendous amount of attention from the males. The courtship may be quite intensive and the males may injure the female ... or, if the size difference is extreme, the female may get fed up with the constant nagging and injure or even kill the males. :-( So you'll want to consider carefully if you're going to mix the sexes.
Finally the last and most important reason why you need to know the sex of your turtle. An obvious reason, but still: Female turtles lay eggs. :-) I talk about this in the video, so I won't get into it here, but if you have a female turtle - of any species - make sure you provide proper laying opportunities for her. Egg binding is a very serious condition and it may kill your pet. :-(
10 minutes ago