i know there's lots of running jokes/theories in the fandom about how many kids joe has fathered, and while i enjoy those conversations, sometimes i think about joe possibly being infertile.
his entire life, joe wanted to be a dad. he's so good with kids, and kids adore him! he'd take up babysitting jobs in high school to save up some money. parents absolutely put their trust in joe because he somehow could handle even the rowdiest of kids. it's like he was made to be a dad... but somehow he was also, quite literally, made to not be a dad.
while 100% infertility can be rare in men, perhaps joe found out in his late teens/early twenties that it would be extremely rare for him to conceive. and when he found out that it crushed him. of course joe kept this desire close to himself, and played it off. "well now i can have sex as much as i want. there's no risk!" and so he doesn't just that. cause hey, sex is pretty great. and of course he would still practice safe sex because pregnancy isn't the only risk of unsafe sex. but at least he wouldn't have to worry too much about a condom breaking.
and so the desire of being a dad was pushed down deeper inside of himself. and as he got older and watched old friends and school mates have children, he would try to play it off. "when are you getting married, joe?" "don't you think it's time to settle down and have a family?" and he would play it off "i just don't wanna be tied down" "i don't think i'm made for that kind of commitment" but really that's all he's got wanted.
and no, not everyone wanted to be a parent. but that conversation with dating was a hard one to bring up. when was the best time to discuss it? too soon and you might scare someone off. too late, and well, if your future desires didn't match it would hurt like hell. rather than dealing with tiring and exhausting conversations, joe decided to live his life alone. or at least with someone new every now and then. casual as can be.
and here come reki and langa. two teens who find themselves getting into trouble occasionally. immediately joe feels like he needs to protect them as if they were his own. making sure they don't get hurt, and if they do, making sure they get the treatment. he tells them corny jokes that only dads find funny. he protects them. he cares for them.
eventually langa and reki get married and have their own children. and joe, of course, is right there for them. one night their baby is crying, and langa and reki cannot for the life of them, calm her down. they're so exhausted being new parents and how the fuck do people do this?! and who's there to help? uncle joe. immediately upon being held by him, being cooed and soothed by uncle joe, she stops fussing. "how did you do that?" and joe just casually mentions some tricks he learned over the years of babysitting on how to soothe a fussy baby.
as their baby grows, uncle joe is always there. for every milestone. every birthday, every graduation, every special event, along with both of her dads, there is uncle joe. and even though he never got to be a dad, through loving her, he was able to fulfill that piece of him. because being a dad isn't all about blood. it's about who protects you, who tells you stupid jokes, and who is always going to be there for you.