As for why Penny never would have mentioned, during all the time she and Leonard dated, that she didn’t want kids, that’s also pretty easy to figure out.
Meaning, of course, that big, future life plans (which had never been her specialty anyway) likely didn’t enter her thoughts right before saying “I do.”
Most aspects of her life had been in flux for many years.
Marriage in general wasn’t really on her mind, much less the idea of kids, so when she realized she was in love with Leonard and agreed to marry him she was probably just glad to have a nice guy who really loved her.