There  is a unique word here, for Jesus, translated as "cleave" and "join." Of course, the word is funny. It means "glue."  The word is used in a variety of contexts in the Greek OT, but none of them related to a husband and wife. The most common, on the positive side, being glued to the Divine and on the negative, being glued to a skin disease. The perfect analogy for marriage.

The word translated innocuously as "leave" is also a rare word, used only three times by Jesus, with the specific meaning of "leave behind."

Though this verse seems to describe marriage, but it may have more the sense that a man and a woman will create a physical body that combines them both, that is, a baby. However, we have to read the verse in the order that it was spoken, as a setup and punchline, ending "the two will exist for/into flesh," a pause here is suggestive because the word "flesh" means "nature taking its course." However, it is followed with the punchline "one." The "one" is not only suggestive but changes the "flesh" to mean "body." Both the Greek words meaning "creating" and "beginning" also point to conception.

The verb used is not the verb of becoming, indicating a change in the man and woman, but the verb of being in the future. In English, we use the future of "to be" to mean "to become," but the word for becoming is a special word in Biblical Greek and, in many ways, is used as the opposite of "being." Perhaps, "will exist" is better in Greek translation to escape the sense of "becoming."