The Al HaDaf brings the Chacham Tzvi, based on the Tosfos on Yevamos 2a regarding achos isha, asking: since every yavam in the world is automatically an onen at the moment of his brother's death, and an onen is Patur to perform mitzvos, so why is there any case of yibum, ever. His answer is that chalitza is not a mitzva, so at the moment of death yibum/chalitza are not actually Patur. Why doesn't he just answer that aninus is more like niddah (in the same tosafos) than it is like achos isha. Just as by niddah, who is equally asurah to all men in the world, not specifically the yavam, so also the yavam, as an onen, is equally asur to all women in the world, not specifically the yevama?
Marc Diamond, Providence, RI, USA
The Chacham Tzvi did not ask that the Onen is Asur, rather that he is Patur. It is possible that your answer is correct as far as the Isur is concered (although there may be other answers as well).
Dov Freedman