The thing is that while there are people who aren't having children because of fertility issues, for lots of people it's because they don't want them, or don't want them yet. So yes, it might be a much less life altering choice than a pet or a house, but it is often a choice.
I think because some people do struggle with infertility, they forget that a lot of people without children are that way because they just aren't ready. This assumption that the only reason people don't have children is because they have issues is, in my opinion, false.
But, like I said, I get that it's not considered okay here so I don't ask. Where I come from, it's a completely normal question and is considered showing an interest, not deeply probing. Although having said that, like anything, that assumes that the person asking is also sensitive to the answer ie a woman who happily tells you that she's not having children because her and her partner are enjoying travelling the world needs a different response to the woman who mumbles, "maybe one day" and moves the conversation on.
personally, I don't understand this obsession with not asking questions in case the answer is something bad. I think it makes more sense that if the answer is something uncomfortable, we learn to deal with how it's talked about. The secrecy around this kind of thing just makes it worse - women accidentally hurting their friends' feelings because they're so excited about being pregnant because they didn't even know that the friend has been trying etc.