OK time for me to dive in !!
The situation here has
nothing whatsoever to do with adults or consent. It has to do with
incest - which is illegal (at least in the UK). Morality is not the issue or whether government have the right to legislate.
Incest has been outlawed because, I think, of the genetic problems that might arise and also, as mentioned earlier, to prevent 'grooming'. IMO quite rightly. Anyway it is illegal and as such these two committed a crime in the eyes of UK law.
I recall a while ago about a man (in Germany ?) who advertised for someone to kill him. There was obvious consent, but IIRC the killer was arrested, tried and sentenced for a crime. I await to be corrected, but I think that was the outcome.
With regard to Incog & OB's comments, I know that if the roles were reversed and the father had had sex with his daughter, the father would most likely be still locked up now ! The law and society in general would have viewed it as a much more serious offence - even with apparent consent.
