A few facts in this case might change some people's perceptions of what they think happened, as it has not been made clear in the Paris Lees blog which Martin linked above; a blog which also does not tell the whole truth of the situation:
1) the appellant, Justine, was 13 years old when she made contact with M on the internet, M was a year younger.
2) They engaged in webchats etc for three years before agreeing to meet under the supervision of M's mother in the latter's home.
3) For three years Justine pretended to be 'Scott' and on the occasions when they met in M's house, when Justine was 17 and M 16 sexual activity consisting mostly of penetration of M took place; 'Scott' regularly wore a strap-on and may or may not have been, in fashion terms a 'goth' (this seems to be disputed).
4) There is some dispute as to whether or not Justine revealed to M that Scott was a girl before the Mother confronted Justine/Scott with the truth -the reaction of M was so severe it is believed she could not have known before the revelation that Scott was in fact female.
5) The legal issue is one of consent, which is why Justine/Scott was tried for sexual assault, and is complicated in this case because of the deception where in case law different modes of deception have led to successful prosecutions in some cases but not others -for example when someone, A deceives partner B as to their HIV status-another example in the appeal courts decisions related to the Julian Assange case where, it is alleged, sexual intercourse only took place on the understanding a condom would be used.
6) the key element of the judgement is M's right to consent to sexual intercourse with someone of her choosing:
Thus while, in a physical sense, the acts of assault by penetration of the vagina are the same whether perpetrated by a male or a female, the sexual nature of the acts is, on any common sense view, different where the complainant is deliberately deceived by a defendant into believing that the latter is a male. Assuming the facts to be proved as alleged, M chose to have sexual encounters with a boy and her preference (her freedom to choose whether or not to have a sexual encounter with a girl) was removed by the appellant's deception. (My emphasis in bold).
To recap: the case involved a 17 year old and a 16 year old, one deceived the other over a period of three years and in physical encounters, we are not dealing with mature adults who met each other in a bar or online and agreed to have sex- there was a romantic element to the relationship between Justine/Scott and M where the element of deception, I suggest was qualitatively different from a hook-up.
The full judgement of the Court of Appeal is here:
http://www.bailii.org/ew/cases/EWCA/Crim/2013/1051.html