I must confess that as a medic I used to always interpret the word “straining” as exerting abdominal effort to expel stuff, rather than the act of filtering. Yet more evidence of my silliness, I suppose.
Straining at gnats while swallowing camels is actually a wry remark about the human fault of obsessing about details while ignoring the big picture. Our Lord was remarking on the pharisees’ emphasis on slavish obedience to written rules, while ignoring far greater infractions of the living natural law. For instance, unkindness is a big sin, yet is common among religious folk.
Our Lord instructs us to be as gentle as doves, and as wise as serpents. We must keep our venom under control, for Christ’s sake.