now that i think about it Joshua and Teru never had a kid. What happened there?

I think that because they are engaged they know for certain they will stay together so there is no reason for them to rush and have a baby. So Teru probably did not continue with the parasite insertions

Not all joinings can produce a baby, even in a male/female relationship. I wouldn't discount Joshua's persistence in trying to get the one he loves prego.

Don't forget, it took a while for the mongoose and snake to get prego, as it was revealed someone kept inserting parasites into the uke persistently.