Shouldn't ethylene should be more acidic? It has a C=C bond, so the density of electron cloud would be higher in the areas of the double bond while the C-H single bond have way less electron density. This, plus the fact that carbon has higher electron negativity than phosphorus should mean ethylene is more acidic as C-H bond will be more weakened than P-H bond and thus, C-H bond will break relatively easily, releasing H+ ions more frequently.
However that seems to be not the case (as discussed in this question Is ethylene more acidic than phosphine?).
Why so? Or am I wrong? Any help or discussion is welcomed. Thanks!