I never thought of Nermal as being male cos I used to read to comic strips before I saw the cartoon. I still don't understand why they used a woman doing a very high voice for him.
Tweety was always a mystery tho but that may have been down to the limited vocal talents of Mel Blanc and the fact the Tweety's voice was just a sped up version of Bugs with a speech impediment. He was only supposed to be three and a half years old anyway.
Also, don't forget, Kermit was having his own gender issues back in the 60s. Remember his blonde wig and miming to Rosemary Clooney's "I've Grown Accustomed to Your Face"?