Can a man ask a female artist to touch his genitals to study nature

No, that would not be appropriate. Consent is crucial in any interaction, especially when it involves personal boundaries and intimate contact. Asking someone to touch you in such a manner, regardless of the intention presented, is inappropriate and could be considered harassment. It's important to always respect others' boundaries and to engage in discussions or studies of nature in a respectful and professional manner. If you have questions about nature or art, there are many ways to explore those topics without crossing personal boundaries.