Joseph wrote: “‘Even to speak with her when she is behind a fence is not something we tell him to do. He should die rather than speak to her from behind a fence.’ This Rambam is talking about even an unmarried woman whom he lusts.”
The case here is of someone who is sick due to lust for a particular woman, and the doctors say that he can only survive if he gives into his lust by conversing with her. There are two opinions in the gemara from which this halacha is based. (1.) Chazal were talking about an eishes ish. (2.) Chazal were talking about even an unmarried woman.
According to the first opinion, it would be possible to say that the reason is that it is stam a case of “yehareg v’al ya’avor” due to abizraihu of arayos. However, according to the second opinion, that cannot be a factor. The reason why chazal wouldn’t permit an unmarried woman to be used in such a manner was so that B’nos Yisrael wouldn’t be treated as hefker – as some sort of object available to cure men of lust. Treating women like that would ultimately contribute to a proliferation of pritzus in society. (There is also a concern for p’gam mishpacha).
The Rambam quoted by Joseph paskened like the second opinion.