A. According to my understanding its more of a mitzva to have more after a boy and a girl not a chiyuv.
B. If it's a issue of hz'l why isn't pregnancy the same issue?
A: there are many opinions. But even if its "only" a mitzva kiyumis, (not chiyuvis), its still called "problematic".
B: there are different types. I just said that there could potentially be issues with some forms. Not that anything that can't lead to pregnancy is h'zl.
The main issue is A.