I don't find it gross if I eat something that someone else took a bite of. Of course I don't go around eating everything I find not finished, but if I know the person, I find it OK to share food this way.
No..I cant eat someones left over. I can even share my glass even to my sister. I just dont know. If they come to me and have a bite. I just give it to them. I dont know but thats me and nothing can ever change that.