Read any comic from the 90s and you'll see Peter was not only encouraged by the idea of being a dad and assured MJ they'd make the best parents in the world, but was fully prepared to become one during the Clone Saga...and, depending on what you consider the 'real' canon, he rose to the challenge aptly.
Post-OMD Peter is a variant corrupted by devil magic. Nothing he goes through in regards to lack of growth really 'counts'.