LOL!...I can agree with your statement about vegans. I was only using them as an EXAMPLE of people having the FREE WILL to choose. So MY point is still valid!
Actually, you just helped to prove my point further. Since according to you part of the natural influence of "stomach flora" in our own body would say eat the meat. They use their own will power to overcome that influence. Because they chose to. WE make choices based off of our understanding and parsing of information that is given to us. Still a choice. An influence is ONLY an influence NOT a command. So that once we hear or see something we somehow become a slave to it. Humans still have the free will to choose for themselves how they will respond.
“Forces beyond your control can take away everything you possess except one thing, your freedom to choose how you will respond to the situation.”
― Viktor E. Frankl https://www.goodreads.com/author/quotes/2782.Viktor_E_Frankl
Our bodies are designed to eat meat. they don't have to eat meat :)