It is indeed objectively wrong to receive Protestant communion. If done with full knowledge and full and free consent, it could be a grave sin. The reasons are because Protestant communion is invalid and because reception of communion in a Protestant church implies a sacramental unity with that church that does not exist. Whether or not a Catholic also meets his Sunday obligation to attend Mass makes no difference as to whether he can receive Protestant communion. Missing Mass would be an additional offense but having gone to Mass does not wipe out the offense of deliberately receiving Protestant communion.
