If having children is a chiyuv then why is it so unacceptable to say it's not enjoyable (at times)? Fasting on Yom Kippur is unpleasant, and no one looks at you reproachfully when you say you are feeling weak and hungry. They don't say "How could you be so ungrateful? Don't you feel spiritual?!"
There is something about the chiyuv to have children that makes people say that every moment is a joy and pure unfiltered virgin nachas.
u have a point on the chiyuv aspect but there is also a natural inclination to want children (not only in the religious world-although more so it would seem) and therefore your'e parentheses is very important many mitzvos don't seem enjoyable at times.
I recently overheard a conversation between 2 wealthier people. One was saying to the other that he has a hard giving tzedakah to "these" organizations (ie: bonei olam, A time) bec. after the parents have the children it's just a burden on them so he feels like is placing the burden on them and what for. he said just leave it up to Hashem if He wants to give He will. (as an aside it is also true that things that come with a lot of effort are generally appreciated more)
But this just points to the natural inclination to have children and be osek bikiyumu shel olam . If it was just for the mitzvah then a chomesh is where the chiyuv stops. on treatments it goes way above a chomesh.
sorry about my ramble-just some of my thoughts on the general topic-take it or leave it (or delete it

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