From a historical context, there may be an additional explanation to the specific use of words in the novel.
The use of "sin" as a euphemism for homosexuality (namely, the act of sodomy), is a product of Wilde's time. The very Victorian "social purity movement" was one of many efforts to limit, if not suppress, the use (or abuse) of women, particularly very young women, for the purposes of prostitution. For this reason the Criminal Amendment Act of 1885 was passed, attempting to curb these activities.
"An Act to make further provision for the Protection of Women and Girls, the suppression of brothels, and other purposes"
It was a member of parliament, Henry Labouchere, who extended these practices to any age group, and not just minors. This revision is found in Section 11 of that same Act. Hence, any act of what was later known as "gross indecency" was considered punishable, whether between men and women, or men and men, adding
a term of imprisonment "not exceeding two years", with or without hard labour, for any man found guilty of gross indecency with another male, whether "in public or in private.
Oscar Wilde, during his own three trials for gross indecency in 1895, admitted that his mentor, Walter Pater, had made suggestions in terms of the use of terminology in The Picture of Dorian Gray. When you think about it, The Picture of Dorian Gray was published merely five years after the Criminal Amendment Act of 1885 was passed. This means that this Act was passed DURING the time that Wilde was writing the novel. Hence, Wilde and Pater alike must have been very aware of the danger of being too liberal in expressing their thoughts and feelings about the subject of "Greek love" through the characters.
For this reason, Wilde may have gone back to review his language, in order to make it more acceptable for the audience for whom he was writing: one which was as puritanical as it was hypocritical. Unfortunately for Wilde, The Picture of Dorian Gray , due to its notoriety, was the FIRST piece of evidence that was used, almost six years after its publication, as the smoking gun that would have shown that Wilde was, in fact, guilty of conducting acts of gross indecency. He was hence sentenced to two years with hard labor at Reading Gaol, under Section 11.
The Picture of Dorian Gray by Oscar Wilde was written in the Victorian period, during which, by most Christians, sodomy (the term used in the period) was considered a sin on the basis of its designation as such in both the Old and New Testament. Rather than the law and concept of sin focussing on the notion of a "homosexual" person, they instead addressed specific acts, and condemned people who performed those acts.
"The only way to get rid of a temptation is to yield to it. Resist it, and your soul grows sick with longing for the things it has forbidden to itself, with desire for what its monstrous laws have made monstrous and unlawful." - Oscar Wilde, The Picture of Dorian Gray Ch. 2
This may refer to sodomy, but could refer to many other forms of sin as well.
For use of euphemism, in part this was simply a case of formal Victorian manners tending towards euphemism, but also a stylistic device that makes the sin more terrible by the sense that it is too awful even to be named.