People who view sex as a causal physical sport, why does your stance change suddenly after marriage and in choosing monogamy?

I (27m) married my wife (23F) as a virgin and she claimed to have no past. Later found about her casual sex chat history with pictures. She defended it by saying sex is as simple as a joyful physical sport with someone and it meant nothing to her. I love her and I accept her past. But I felt lot of FOMO of missing this "causal" part of human experience, forever in my life. So I requested her to let me experience it also now and let me also equal her body count of 6 and I'll do it just as a physical novelty. Suddenly, sex means more to her and she cannot let me do it. It feels unfair.

Question to people (both men and women) who see causal sex as just physical sport. Why do you change your stance after marriage and why you insist on monogamy for both partners? Genuine question to understand human psychology as a person who never experienced casual sex.

TLDR: I just want to understand why some people view sex as casual before marriage and something so sacred between couples after marriage. Honest question to understand psychology.