Injecting a foreign substance into your body without consent, does that sound legal ?

Its not that clear: https://upload.wikimedia.org/wikipedia/commons/5/50/Stealthi...

I'd argue you should not give implicit consent to insert a syringe in your body if you don't want a foreign substance injection. Now all this would be solved if consent was explicit, but how explicit ? Should we require a signed contract before intercourse ? in fact that's how it's supposed to work in a lot of countries. The contract is called Marriage and intercourse without marriage is forbidden