A.T. Robertson Commentary Matthew 19:5

"and said, For this cause shall a man leave his father and mother, and shall cleave to his wife; and the two shall become one flesh?" — Matthew 19:5 (ASV)

Shall cleave (κολληθησετα). First future passive, "shall be glued to," the verb means.

The twain shall become one flesh (εσοντα ο δυο εις σαρκα μιαν). This use of εις after ειμ is an imitation of the Hebrew, though a few examples occur in the older Greek and in the papyri. The frequency of it is due to the Hebrew and here the LXX is a direct translation of the Hebrew idiom.