The original definition was by implication homophobic; the term 'unnatural' has a long history in legal and biblical commentary deriving from Romans 1-18-32, in which the apostle Paul condemns non-procreative sexual acts. Of course, in 1899 male homosexual practice was illegal under UK law, but this was also the case in 1933, when the OED1 Supplement editors produced a much more neutral definition of the term homosexual itself, included in the OED for the first time at this date: 'pertaining to or characterised by propensity for one's own sex'. It is interesting that the OED1 lexicographers did not supply more recent, post-17c, evidence of usage for catamite, since they themselves used catamite as a definitional term (for senses of Ganymede, hermaphrodite, ingle, and elsewhere) and it would not have been hard to find examples elsewhere (e.g. in Robinson Ellis's commentary on Catullus, published in 1876). [1]

Given that the editor of the 20th-century Supplement to OED, R. W. Burchfield, took a special interest in updating the treatment of sexual vocabulary, we might have expected him to have revised the entry and removed the condemnatory term 'unnatural' - especially since he too used catamite in some of the definitions he added to the Dictionary (e.g. for jocker, 'a tramp who is accompanied by a youth who begs for him or who acts as his catamite'). But he left the OED1 entry as it was, neither re-writing the definition nor adding more recent quotations of its use.[2]

Burchfield was no lexicographical homophobe: he was the first to include the sexual sense of lesbian and related words in the OED, and he carefully rewrote the entry for homosexual (adj.) which had only been added to the Dictionary in 1933 (he re-phrased the definition and supplied further quotations, also adding definitions and quotations for the nouns homosexual and homosexualist). However he was inconsistent in his updating. Not only did he leave the term 'unnatural' in the entry for catamite untouched, but he also passed over OED1's definition of ingle (sense 1) as 'A boy-favourite (in bad sense); a catamite', though he certainly considered the entry since he added two quotations to it - from T. E. Lawrence (1926) and H. Nicolson (1962). Similarly, Burchfield let stand OED1's definitions of Sapphism as 'unnatural sexual relations between women, and tribade as 'a woman who practises unnatural vice with other women', while adding sapphist and sapphistically to the Dictionary along with accompanying 20c quotations. 

These editorial decisions tell us both about Burchfield and about the culture of the time; they indicate that different sorts of homosexuality, whether male or female, continued to be problematic, in varying ways, into the 1970s and 1980s. Like the original OED1 definitions, Burchfield's treatment of sexual terms constitutes lexicographical evidence for these attitudes - as does also the decision of the OED2 editors, in their merging of OED1 and the Supplement in 1989, to rewrite the definitions for Sapphism and tribade, dropping the references to 'unnatural' and 'vice' (though the term 'vice' lives on in the etymology of Sapphism even today, i.e. December 2011).

OED2's 1989 tolerance of the OED1 definition for catamite, with its reference to 'unnatural purposes', is likewise an important piece of historical evidence. And so is the recent decision, apparently part of the OED2 re-launch, to re-write the definition and re-date the last quotation from 1795 to 1822 (see discussion on previous page here). That is why it is imperative that the lexicographers should date the change clearly, alerting users to its much more recent provenance.

Finally, it should be noted that OED Online has been inconsistent in this unlabelled partial updating. The Dictionary continues to contain other definitions and editorial material indicative of cultural attitudes to sexuality or other matters which are today unacceptable - as in the entry for ingle, quoted above. And 1822 is certainly not an accurate indication of last use of catamite: google is a ready source for one of the most famous recent instances of the word, the opening sentence of Anthony Burgess's 1980 novel Earthly Powers: 'It was the afternoon of my eighty-first birthday, and I was in bed with my catamite when Ali announced that the archbishop had come to see me'.     

Footnotes
[1] Ellis 1876: 76. Ellis was a close friend of James Murray, the chief editor of OED1; see Murray 1977: 324-5; Brewer 2007b: 175.
[2] For Burchfield's interest in sexual vocabulary see Burchfield 1972 and Brewer 2007b: 181-2, 203-5.  catamite was used by Aldous Huxley, a writer intensively quoted by Burchfield in the Supplement.