Was this an irresponsible way of describing the difference to an 8 year old?

No, it wasnt, and if your wife has a problem with how you discuss "big" topics with your children, maybe she should have married someone else. Sitting around waiting for your partner to get home to discuss everything makes it seem as if you're not capable of having an independent opinion. At 8, I wouldnt have gone into any detail about how the "parts" fit together, simply explained that sometimes people love someone of their own gender, kids tend to follow up with more questions if they're not satisfied yet, I normally start with a very simple explanation and give more detail if requested.