en_tn_condensed/mrk/10/08.md

For this reason a man ... one flesh

Jesus continues to quote what God said in the book of Genesis.

they are no longer two, but one flesh

This is a metaphor to illustrate their close union as husband and wife. Alternate translation: "the two people are like one person" or "they are no longer two, but together they are one body" (See: rc://en/ta/man/translate/figs-metaphor)