“Venus pudica” pretty well covers it. (Rimshot.) There’s no need I know of to explain the meaning of a pose that all art historians are familiar with.
If you must use a term, then “crotch” is acceptable, as are the clinical terms like “pudenda”. “Private parts” might work in a pinch. I like the sound of “labia” myself.
And if you trust your advisor, even the brisker terms are possibles. I used the word “prick” in my dissertation, but it was a quote of somebody else.
Personally, I was always a bit adventurous in college, so I’d use pussy, snatch, or cunt with a little explanation of you plight in a footnote. My teachers would have found it immensely entertaining. In this same vein a comical euphamism would be ideal. I guess I just like to think teachers have a sense of humor and appriciate it in your work. The one exception is if its a class where your are explicitly graded on your composition, such as a English Language or Composition II class. I would frequently use colorful and juvenile phrases in my Literature and other Liberal Arts classes. I’d wing it!
One suggestion is to use something such as pubic cleft, its somewhat clinical without being sanitary IMHO.
Boys, you really should take a look at that picture, its not what you think.
The woman is resting her hand in her lap. That’s all. She has clothes on there for pete’s sake. The artist didn’t want any depliction of genitalia there so she is just resting her hand in her lap.