Streamlit magic displays your string as a markdown text, and if you wrap some markdown text between _two underscores_, it is generally displayed in italic.
You can either add a backslash before gender’s underscore…
s = "(full_name, birth_date, gender\_)"
…wrap your string with backquotes (which will render with a monospaced font)…
s = "`(full_name, birth_date, gender_)`"
…or use st.text to display your whole string (renders with a monospaced font as well):
s = "(full_name, birth_date, gender_)"
st.text("string: " + s)