Thats because men dont think about it on the level women do, women feel its almost expected of them to have children, you need to stop feeling that you have to fit in to societys expectations. Also sometimes women do question it more as their biological clock starts to tick.
Men can father children much longer, where as a woman only has a short time to pass on her genes! so I can understand why men can get past it easily
There are loads and loads of women who remain childless, maybe you need to join a childfree by choice forum, these women will be in the same position as yourself and can give you support.
I obviously mistook your last post, I thought that you were confused as to whether you felt that you wanted children or not. Obviously this would be an issue as your husband is 110% certain!!
here is a link to a board thats for women who choose to stay child free.
http://messageboards.ivillage.co.uk/iv-ukntcfb c/