Your husband is right, he is way too young and you are right, you need to be open about ALL subjects with your children.
Sounds like your son is a lot like my oldest boy. Everything (especially scientific things) needs an explanation and you can not give a quickie version and satisfy them. What I did was explain that moms are built with eggs in their belly that can grow babies. When it is time for the babies to come out then mom goes to the doctor and has him/her take it out for mommy. As I am sure you guessed, this was not enough information for my son, he asked how the baby got in there in the first place. We believe in a Christian God so I explained that God decides when it is time for the baby to be there. He of course prodded further and I simply said that he needed to be a bit older for more detailed descriptions. He did ask a lot about how the doc gets the baby out and if it hurt and was there blood and to that I answered "yes". When he pushed too far, I would explain again that he needed to be a bit older to understand more detail.
In short, you are right, you need to be open with your kids so they can take on your values and have the right information to make decisions for themselves. The problem is, at 6 1/2 they may seem to be able to handle all this information but they really cannot. They have a hard time making the connections of love and sex that they should. Also at this age, they tend to share EVERYTHING with their friends. I don't know if you have ever heard your 6 year old translate what you have said to their friends before but rest assured it comes out differently then how you said it. Not to mention the fact that now a bunch of 6 year olds are running around having sex talk with each other and no ability to truly get it.
Just keep it light, real and surface. At about 8 1/2 my son read all about sex in a science book we had and then informed me he knew everything. Which of course he didn't. My window was open to talk about it, direct him and clarify and for the record, at 9, he is still not totally equipped to handle it all but we have a start.
Good luck and tell your husband the he is right about the age appropriateness of this subject but needs to get over his fear of raising curiosity. Kids have a funny way of running out and finding information they aren't getting at home, so by not talking about it, he could chase your boy into exactly what he didn't want - experimenting.