No, as far as I know, "πράγμα" does not mean "love within marriage" in ancient greek.
Some of the most common meanings of “πράγμα” in ancient and modern Greek are “thing”, “fact” or “circumstance”.
“Οι γονείς του Γιάννη αγαπιούνται πάρα πολύ. Αυτό είναι σπουδαίο πράγμα σ’ ένα γάμο.” = “Yianni’s parents love each other very much. This is a very important thing in a marriage.”
Therefore you might have seen something similar to the ebove example.