How you handle it really depends on how you, yourself, view sex. Not talking about it at any age doesn't really help anyone, because they'll just find out about it from friends/family in a way that you might not be comfortable with.
Its our jobs as parents to prepare our children for life. And unfortunately, that means talking about embarrassing things like this!
I think sex is a very beautiful and natural act between two people. And I fully acknowledge that you do NOT have to be love to have sex, that is just how some people prefer it. So, I will tell my kids that it is something that carries a lot of responsiblity with it and therefore, adults are much better equipped (literally!) to handle it and its repercussions.
I think you need to be age-appropriate with your son and not give him more information than he needs or wants. For example, my daughter is 5 and knows the proper words for genitals. She knows where babies come out from. But she has never asked how they get in there and I hope to wait when her natural curiosity takes hold first. If and when she does ask, I'll probably explain it to her simply without mentioning intercourse first. Then, when she is developing into a mature woman, I'll add to our discussion and give her a few more details and answer any questions.
Good luck! Its hard when your kids are exposed to something before you are ready for it!