As long as who you see is the same sex as you, and you don’t have same sex attractions, then no it is not sinful. If you do have same sex attractions, then I’m not sure. Although it would definitly be a sin to think lustful thoughts about people you see there.
No as long as you aren’t lusting after those people. It is also not a sin to take a life drawing, sculpting, ect. class as long as you aren’t lusting. The human body is a beautiful thing. It is a part of God’s beautiful creation.
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