1 Samuel 18:1-4Using these verses as part of the marriage service implies that David married Jonathan (and thus became Saul's son-in-law) through this covenant relationship.
And it came to pass, when David had finished speaking to Saul, that the soul of Jonathan was know with the soul of David, and Jonathan loved him as his own soul. And Saul took him that day, and would not let him go home to his father's house. Then Jonathan and David made a covenant, because he loved him as his own soul. And Jonathan stripped himself of the robe that was upon him, and gave it to David, and his garments, even to his sword, and to his bow, and to his girdle.
But this is contradicted by verses later in the same chapter. David resists becoming Saul's son-in-law when Saul offers his eldest daughter Merab.
1 Samuel 18:18
David said to Saul,"Who am I and who are my kinfolk, my father's family in Israel, that I should be son-in-law to the king?"
Later in the same chapter, when Saul wished to offer his daughter Michal to David, David still resists because of his low social status:
1 Samuel 18:23 So Saul's servants reported these words to David in private. And David said, "Does it seem to you a little thing to become the king's son-in-law, seeing that I am a poor man and of no repute?"Clearly, the covenant between Jonathan and David did not make David a son-in-law to Saul. So, the progressives are using the verses in a marriage ceremony as a proof text:
Prooftexting is the practice of using decontextualised quotations from a document (often, but not always, a book of the Bible) to establish a proposition rhetorically through an appeal to authority. Critics of the technique note that often the document, when read as a whole, may not in fact support the proposition.
I t appears that when the verses out of context serve the progressive cause, they are not averse to ignoring the context.
When I read the first verses of the chapter, I think that Jonathan is making the gifts of his royal clothing and weapons to David to show his acceptance of David as an equal. In this reading, the subsequent issues David raises regarding his inferior status make sense.
H/T Baby Blue