the gemara wanted to prove from yosef that one can't have relations during a famine. but before his children were born yosef had none. so for him it would have been permitted to have relations during the famine. so what is the gemara's proof from yosef. is it a kal vechomer??
yisroel merkin, jerualem, israel
Dear Yisroel,
Even as is, Yosef should be permitted since he has no daughter though some disagree.
However, Yosef acted with extra Chasidus even before he fulfilled Piryah ve'Rivyah. The Gemara's proof is that it must be forbidden at least for someone with children. Otherwise, Yosef would not have acted with Chasidus (see Maharsha).
Tosfos learns that there is no Issur for anyone at anytime, only Chasidus. And Yosef went a step further. We learn the Chasidus from him, at least for those with children.
Gevuros Ari (from the Sha'agas Aryeh) explains that the phrase "be'Terem..." is not applicable to Yosef. Because its extra, it teaches us about someone with children. (See there for another example).
All the best,
Reuven Weiner