First of all, your Pew poll is from last year. Here is a more recent one.
Regardless of whether or not you think it should be legal, please tell me whether you personally believe that in general it is morally acceptable or morally wrong to change one's gender.
Acceptable 43%, unacceptable 55%.
Pew and Gallup are also asking different questions. It is possible that someone thinks gender transition is immoral, but at the same time a trans person should not face discrimination. (Just because someone has done something immoral, that does not automatically justify discrimination against that person.)