Hey Hannah, if you're reading this... wake up! You only want a baby to distract from your issues. Shane will be jealous and it will ruin your relationship as he's immature, fundamentally way more selfish than you, and you've conditioned him to have you waiting on hand and foot at a moment's notice. You will grow to resent him as he demands your time be split between him and the baby. He is basically already telling you this so listen. You are only 24 and have barely experienced life: face your issues and try to grow as a person first instead of always trying to fill the void or distract yourself with obligations.
And FYI, it is not a guarantee that just because the baby is HIS as opposed to a puppy, he won't get jealous. It's a very common situation, even for men that aren't as selfish or dependent as Shane. Also, sometimes it takes men longer to bond with their child and Shane can't even hold the baby! It could take him a long time to really, truly bond with the baby and be ok with the baby taking up all the attention.
Shane isn't ready to have kids? But... he won't be doing ANY of the work. And like... his lifespan estimation is considerably shorter than average, if he plans to have kids he should be wanting to do it earlier rather than later. Is he delaying having kids because he's worried Hannah won't have enough timr to cater to his every need?
A baby being around would affect his life minimally, it's not like he's out barhopping every night! Also Hannah is CRAZY. The girl is what, 24? And dying to have a baby while full time caregiving a grown man AND a dog?
A baby being around would mean that Shane doesn't get to be the baby anymore. it means that he would spend way more time with a caregiver than with Hannah.
And deep down, I think Shane knows that Hannah is with him in large part because of her obsession with taking care of someone, and a baby would mean that he gets replaced as the obsession.