Nice idea but 1. How do you prove they ingested an illegal substanceand 2. What substances will be illegal? Any food in too much quantity can cause health problems. Is fast food legal or illegal in your world? Tobacco? Wine? Processed food? Beef? Sweets? The list goes on.
The thing about laws that you seem to have a very difficult time grasping is that you cannot just make up laws that suit your POV. The law must be specific. If you want to deny service because of something they ingested you have to prove fault. Kind of hard to do. Some tbings are easier tonprove (meth or hard drugs) but they are a small portions. Tobacco kills roughly half a milion peolle a year. Im all for denhing them public services but good luck with that.
That is all beside the point. The issue is freedom. You and the state have absolutely no right or justification to tell me what I can or cannot do with my own body. So long as there is no dirrect affect on you, you need to mind your own business.
If two people of the same gender love each other and want to share their lives together itnis none of your business. It does not affect you. It does not involve you. You have no say.