First of, Turing was the most high-profile case, not the only one. And again, the FBI stands to benefit from discrediting Simon because of Bernie. So they are eager to find dirt on him or someone else in Bernie's circle. The story does eventually lead to the two being prosecuted under sodomy law, but there's no chemical castration, just prison. And second, the FBI know Penchek is here because his business with them is only a secret to the public. And his business is not just with them, but with CIA and the House U-American Activities Commission The FBI are involved specifically because they put Bernie in situation, and they're the ones who are best positioned to put it the way the British government wants it, specifically MI6 have an interest in Bernie being spared the blacklist, hence Penchek, though diplomats are also sent
Holy crap, looking at my post in your comment, I really typed that way too fast. Stupid smapl fidlly touchscreen. Also, is that supposed be a positive or negative comparison? Or just funny?
None of the above. The whole "lover who is a subordinate" situation was shocking when Lawrence tackled it, and it would still be shocking in the 1950s. Closer to the OP, you could do worse start with https://en.wikipedia.org/wiki/Mary_Renault for some background into where homosexuality sat in the 1940s/50s.
I don't that woman is a very good example of where we were. She was a lesbian, and lesbians have always been better of in terms of most homophobes because they often have an odd obsession with sodomy, and there is of course the jealousy issue. Plus, Simon is someone people in power want to see prosecuted, so they're more eager to charge him than the average gay.