So, contemplate a person with a severe food allergy. It's not the worst possible scenario (airborne and lifethreatening anaphylaxis) but it's pretty bad: severe hives, respiratory implications, etc. Or maybe it's not a food _allergy_ per se (a protein triggered IgE reaction), but some other kind of food intolerance, and the manifestation is bleeding sores on contact (and let me tell you, bleeding sores on contact in the esophagus and similar is nothing to mess around with. Never mind the rest of the alimentary system).
Let us assume that as an adult, the person with this severe food issue has more or less identified what they absolutely cannot eat, and over time, they've figured out where they can and cannot go out to eat and so forth. They have more or less mastered the intricacies of talking to servers and coaxing the servers into talking to the kitchen -- and they've gotten really freaking brilliant about tracking down online ingredients lists and so forth, and become quite expert at the typical construction of anything they've ever eaten and most of the typical variations. They still are periodically sent to the ER by a blithely incompetent server, or an overly creative kitchen, but they mostly have it under control.
Now let us give this adult a person with whom they enjoy physical intimacy. They kiss. They lick. Maybe they bite. Etc. All over.
I now know two people with food issues who have to pay close attention to what the person they enjoy physical intimacy with is consuming, because that person's mouth is a cross contamination hazard.
This is so incredibly unfair, on so many levels.