“I do not belong to myself, for God bought me with a high price (1 Corinthians 6:19).”
Your body is the temple of the Holy Spirit, who lives in you. God gave you this gift of holiness through the costly shed blood of his Son. Realizing that we are now eternally promised to the Lord gives us a different perspective on how we treat our bodies while on earth.
Helpful boundaries, then, are those that encourage the holiness of each person, protecting the body as the blood-bought temple of the Holy Spirit. There should never be pressure to cross boundaries, nor should the question ever be, “How far is too far?” That question illustrates that gospel-motivation has already been abandoned in favor of selfish ambition.
What is considered most helpful for each person might vary, as some have a weaker conscience than others, but overall, bringing glory to God through our physical bodies should be our underlying motivation and purpose. Christ died so that we would be made holy.