A few things to deal with here, I've quoted KG but attempted to answer the questions brought up by others as well
Kinkygentleman wrote
. The same Act also prohibits a person from having sex with any person who has a mental condition which 'impedes choice'. It does not specify exact mental conditions.
the 2003 act explains sex with someone who is "mentally unfit" at S30. entrire act here, s30 selected http://www.legislation.gov.uk/ukpga/2003/42/section/30
it's lacking the capacity to choose and to be able to communcate that choice.
There are also limits on whom the court sees to be mentally unfit in different statutes, and several tests that can be done before any court case ( remember even if the person is mentally fit it's still rape). the reason it's not described there is because sexual cases are very subjective and it may protect someone who is not protected otherwise.there's also charges against a person in a position of trust which would apply in these kinds of cases.
there's also a whole section on sex in a public lavatory http://www.legislation.gov.uk/ukpga/2003/42/section/71 which replaced a 1997 statute (law) but generally if it is an act capable of causing offense then it's illegal. Offense is to a reasonable person, or as one judge put it " the man on the Clapham omnibus"
sex in public is generally illegal, if you get caught - which in reality depends on where you do it and at what time and basically luck. As for Scentencing it depends on a whole range of things including what you are charged with, who was involved and how severley it's affected people. If you want more info on scentencing ranges the Scentencing guidelines council has a pdf entitles "sexual offenses act 2003" though it is full of legalese
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Adnaw
you are right that Case law does set precedent but it takes a while, something enacted in 2003 won't have a lot of things reported on it, and the courts have operated on similar systems for years. While the law can become more rigid it can also become more lenient and flexible! Precedents dont usually change statue words, just their interpretation and meaning. (it's complicated at times but interesting for historical reasons)
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Sexual offenses 2003 isn't the newest law regarding sex as there's parts of the criminal justice and immigration act 2008 that makes owning "extreme" porn illegal. this coveres BDSM acts that it's legal to do like sarification ( cutting) but illegal to watch. It also makes comic books like watchmen illegal. This law is one that is very contencious but has room to develop in coming years