Sanhedrin 5b - Paskening an Outrageous P'sak

The gemara says that R. Chiya refused to give Rav semicha to paskin on Bechoros because Rav was to much of an expert and would likely permit things that would lead people to compare other blemishes to it, and permit what should be forbidden. The Shulchan Aruch Y.D. 242:10 writes that it is forbidden for a Posek to paskan a p'sak that is תמוה, strange, to the public in a way where it would seem to everyone that he was matir an issur. Gr"a cites our gemara as the source of this ruling, as does the shach (17) from hagahos maymonies. The Shiyurei Bracha printed on the margin discusses whether the nature of this prohibition is that the people will suspect the chacham of making a mistake, or mislead them to make a mistake. He proves from the source being our gemara that the concern is not just a chashad, but that it will lead the public to make a mistake.

In R. Moshe's teshuvos on the topic of artificial insemination (printed in the dibros moshe on kesubos), he was strongly challenged by dayan breish (chelkas yakov) [who he is buried right next to on har menuchos], about this p'sak. One of the challenges was that even if true, it is a p'sak that is strange to the public and shouldn't have been given. R. Moshe responds that the issue of paskening something strange is only when it can potentially lead the rabim to make a mistake and be matir something that should be assur, as in the case of rav regarding being matir bechoros. But, in this case where there is no possibility of leading the public to make a mistake, the din doesn't apply. R. Moshe then goes on to say one of the most classic logical arguments:

וגם לבד זה, הא לא שייך כלל דבר תמוה בזה, דכי עד עתה היה מנהג אצל מורי הוראה לאסור, שהרי רק עתה מעת שנתחדש ענין זה התחילו לדון ע"ז שודאי רשאי מי שסובר שהוא מותר להתיר, ואין להחשיב מה שישנם מורי הוראה אחרים הוסברים לאיסור לדבר תמוה להתיר, שהרי גם אלו הסוברין לאסור יודעין ומבינים גם צד ההיתר ואין להיות להם תימה כשישמעו שיש סוברין להיתר, ואם אין מבינים כלל צד ההיתר אינם מורי הוראה בדבר הזה ואין לחוש להם כלל כי יש להם לעיין בדבר ויראו צד ההיתר ששוב לא יהיה דבר תמוה

Basically, he argues that it is not considered "strange" since even those who hold it is assur certainly see the tzad to be matir, because if they don't see the tzad heter at all, they wouldn't qualify as morei hora'ah and we don't have to be concerned with their opinions at all.