Oh Ninja, I totally agree that secondary infertility seems such a weird concept. Especially as eveyone else looks at DC1 and suddenly feels the need to ask personal questions about having more babies.
I don't think that fertility is a black and white issue, I think there are all sorts of shades of grey and that sometimes we hover between the two labels (depending on age, health, our partner etc. etc.) and that the only thing separating us from pregnancy at any given time could be something as silly as luck.
I think that secondary infertility can happen for several reasons.
Firstly, fertility (in women and men) does decline over time, so what came easyish first time round may get a little harder second time round. That's fine and dandy for super fertile couples, but for people who are little more border line it can push them from fertile to infertile.
Secondly, our health can change and fluctuate. Stress and lifestyle issues may impact on the sperm etc. etc.
Thirdly, it is hard to know what odds were overcome first time round. I have endometrosis. I didn't know that when I was TTC DC1, it took a while but we got there in the end. However by the time we were TTC DC2 there was enough damage (along with the aftermath of an ectopic pregnancy) to mean we had to use IVF. I'm guessing that having DC1 naturally was incredibly lucky - it's just that we didn't realise how unlikely it was when we were trying first time.
Fourthly - sods blood, unfair, stupid law.
Finally, just because we drift towards the infertile label, doesn't mean that at some point we won't drift back to the fertile label in future. There are so many couples who go through infertility and then suddenly, unexpectedly find they are pregnant.
I'm sorry I can't recommend any reading - I never found anything that really helped me.
Good luck with the hospital appointment - I hope you get some answers.