So many ways of looking at this! I disagree fundamentally with Oscar.
Transwomen are not women.
Legally, many trans women are women.
Humans are sexually dimorphic
This is true according to the organising principles by which humans have chosen to interpret biological observations. Historically, this organising principle had the aim of differentiating between men and women in order to control women's sexuality and fertility.
It has be one rule across the board, which means that if 'she' is to be used of any MTT the sentence 'this woman committed rape with her penis' has to be acceptable and meaningful.
Yes, it is meaningful. Moreover, it relies on a locally determined legal understanding of rape. In Canada, for example, there is no crime of rape. So in Canada you'd have to charge the same person with sexual assault.
And why the focus on willies?
Because of the conceptual history of one particular form of sexual assault in which a woman could be impregnated by a man who was not her husband. Historically, this was considered a crime against her husband (or her father, if she was unmarried) rather than against the woman herself.
It's not ok for anyone to rape anyone else. But the way we understand rape is a product of history and culture, and changes according to place and time.
The way we understand sex is also a product of history and culture and changes according to place and time.