Begging for spit in mouth is quite unusual thing in healthy sexual relationship. While deep lip kiss saliva used get mixed by tongue leaking but spiting is quite unusual. Some people have unusual sexual fantasies like nail abrasion,deep teeth mark causing bleed,hunter mark on back,tieing partner on bed with cuff,these all are wild sexual fantasies. Spiting is very awkward thing to do in your loved one's mouth. You can ask her in detail why she enjoy such unusual things. Frankly speaking it's not normal sexual conduct,it's absolutely professional porn thing.