No, you are failing to understand the point. To answer your questions:
1) In order to ask this question you first have to know what the limit *IS*. The Scottish law effectively simply says that the limit is "too fast".
2) You say "the prosecutors don't make the law" but then you give an example which seems to suggest that the Police do! (They don't of course)
3) There are Home Office Guidelines on the amount of drugs above which someone can be classed as having "possession with intent to supply".
4) This is not just distasteful, it is stupid and irrelevant. The offence is you forcing yourself on someone else without their consent, that is the limit which is clearly defined in law, unlike the definition of so-called "Extreme Porn" which is *not* defined.