How to establish expectations without setting a boundary

I’m working through the material and homework on relationships and have a question about boundaries and manuals. If I should not create a boundary unless it is being violated, how do I express something that is zero-tolerance? (i.e. I once had a boyfriend who had used pornography prior to our relationship. When we were talking about what exclusivity meant to us, I needed him to understand that I would consider the use of pornography cheating. He hadn’t violated that boundary, but I felt the need to make that clear so that he didn’t inadvertently think it would be okay as a result of my not stating the expectation, since that’s not an issue for every woman).