1) I think you are ignoring that it is possible for non-parent employees to be negatively affected by the pandemic as well. You seem to be dealing with it very well (which is great!), but others may be stuck working from noisy, cramped apartments and may be getting little to no social interaction. This is can definitely cause health issues and degrade happiness.

Those employees look at the large amount of family leave parents are getting and ask "why can't we get a little extra help as well?". If parents get 10 weeks of leave, how about you give non-parents 1 week of extra PTO or leave. It doesn't need to be exactly equal, but the pandemic is hard on everyone.

2) I think the non-parent employees are saying the company is showing little to no empathy towards their struggles. I really think it's less of "how dare that parent take extra leave" and more of "how dare facebook give parents all this extra leave and give non-parents nothing to help them cope with stress."

3) You choose to have children, but you do not choose to have cancer. I think your comparison there is misguided.