I'm probably the last person you would ask, Graeme, but for some strange reason I seem to be the first to respond. (Edit: Well it seems I'm not the first; I just type slowly.)
Your sons are certainly not too young to have the "birds and bees" talk. Of course they may have had that already or a lot of sex ed in school. In the old days, we learned about that stuff from our friends who had older brothers. If you need to have the talk, you may as well have the whole thing including your additional revelation.
I guess you have to first evaluate where they stand in their knowledge of sex/reproduction/dating and start from there to plan what you will say. You have to decide if this is a father/son talk or does it include mom. Do you talk to each son separately? There are a lot of things to consider. I trust the counselor will have some good advice.
I think the most important thing is to emphasize love...the love mom and dad have for the son and for each other. That's an important thing for them to carry over into their future relationships. Giving them honest answers to their questions and emphasizing the importance of love in their relationships, current and future, will make all the difference.
So, there's probably not anything above that you haven't already considered, but maybe this will get the ball rolling with more substantial suggestions.
Best wishes.