I have no idea what a dental jam is. However, a dental damm is a bit of latex that is used to cover the pussy while doing DATY.  I would never see a lady who makes you use a dental damm.

I regretfully say that i've only seen one once and did test it, not much of a loss in sensation for the woman if there's some lube and the partner is good but primarily used against transmittal of STD's like Herpes Simplex 1 and 2...even Simplex 1 (cold sores on mouth) is transmittable from mouth to there and back over to the gentlemen..eeekk! I've had unsavory gentlemen try to come at me with a visible cold sore on the lip..no way...imagine those that have them IN the mouth...makes you think...before someone goes in for the kill down there, I like to discretely check around with some light kissing plus..it's a scary one out there...just use your judgement, keep egos out of it, and stay safe...