If the idea is a midah k’neged midah, that failing to come to the kohen for the positive end of bringing matnos kahuna will lead to coming to the kohen for some negative consequence, then why illustrate the point specifically with the parsha of sota? Why not the parsha of metzora, which was a severe punishment that required the intervention of the kohen? The Shem m’Shmuel’s answer should be required reading for all married folks and those who plan to be married.
Life is filled with transactions between givers and takers, mashpi’a and mekabel, tzurah imposing itself on chomer. It’s usually very easy to spot which side of the equation is which. Not so when it comes to matnos kahuna. Even though the farmer must surrender part of his crop to the kohen or levi, it is the kohen who is the mashpi’a, the tzurah. The farmer receives far more from the relationship than what he gives up in material goods.
If someone doesn’t get the message and thinks that relationships are only measured by tangible net gain/ loss, if a person thinks he gets nothing in return for what he gives to the kohen and therefore keeps his matnos kehuna, then he will inevitably have a shalom bayis / sota problem to deal with. A husband who, for example, brings home a bouquet of flowers, is not the mashpi’a – he is the mekabeil, because he will receive the benefit of an improved relationship with his spouse that will help be mashlim him as a person. The return is far more valuable than the few dollars invested in the roses that are gone a week later.