With the score at 1-1 just before the break, defender was sent off for denying a goalscoring opportunity when Jordan Roberts tumbled just outside the area.
Yeovil, who also had Connor Smith sent off for two bookable offences, defended valiantly against Crawley’s full card but couldn’t hold on for a point as Thomas Verheydt netted late on.
Boss Darren Way said: “It was very poor decision making from the referee. Watching Omar’s on a replay it was never a sending off.”
If Sowunmi’s appeal is deemed unsuccessful he will miss the visit of Bradford City in the Emirates FA Cup third round on Saturday.