Our 8 year old son saw my wife and I having sex the other day. Now I don't see anything wrong with it, yet my wife is unsure. What should happen here?
You should not make a huge deal of it, but take this opportunity to ask him if he has any questions, reassure him if he is upset, and provide him with age-appropriate education about human sexuality in a low-pressure setting. (There are lots of questions with more information about how to accomplish this available.)
Eight isn't too young for the non-graphic version, and either way I think it's best to have 'the talk' as a gradually evolving conversation over the course of growing up, rather than a confusing and uncomfortable info-dump to a teenager who has already gotten a good deal of (mis-)information from media and peers.
Kids have accidentally seen their parents making their new siblings since the dawn of time (and I imagine with far greater frequency in times and cultures where everyone lived in closer quarters). Unless it's handled with an excess of shame, blame, or general drama, I don't think it's likely to be a major source of problems.