When do you give her enough space or step in and put your foot down?

You are not wrong at all in feeling this way. Just let her know that you are not comfortable with her doing this. If she respects you then she will not do it. If she loved you she will understand. Would she let you do something like this is the real question.

First. No one can tell you what to do with your relationship and your life.
How you feel about a situation is a reflection of what you been through, and should not be ignored.

If you feel as thought the situation is going into a potential cheating one, then your probably right. Tell her what's on your mind and see how she reacts.

Truth is. If she wants to cheat. There is nothing you can do to stop that. She will do what she wants to do. She just will not tell you about it if you don't agree with it.

Ask what kind of posing she will be doing. Who all will be present while she is posing. Where will this posing take place.

There are a lot of questions to be asked for better understanding.
I hope you find your answer.
Good Luck

Sincerely,
A Loving Black Man