Can cunnilingus cause intraabdominal free air?

Yes. There are several case reports.

Mechanism

Other causes of nonsurgical pneumoperitoneum include pneumothorax, spontaneous bacterial peritonitis, peritoneal dialysis, postpartum coitus, vaginal douching, postlaparatomy, postparacentesis.


Source

Cotton, B. et al. "Pneumoperitoneum from Orogenital Insufflation: An Incidental Finding Resulting in Nontherapeutic Celiotomy" J Trauma. 2005.