How do I, as a mother, explain the facts of life to boys? My husband is non-practicing and uninvolved.
I would ask first whether you have brought this topic up with your husband. Perhaps he might respond well and become less "uninvolved" if he were reminded of this responsibility, and you showed confidence in him. If you are worried that he might not say just the right thing, I would remind you that the most important thing is that your boys identify with a man in their understanding of their sexual role.
If your husband really is unreliable (but don't be too quick to assume this; he could rise to the occasion if you gently and lovingly encourage him and don't seem too critical), what about the grandfathers on either side, or an uncle, or, better yet, a godfather?
If your sons ask you questions, your first response might be to encourage them to ask their father. If it turns out that you are the one who must address the question, be sure that you present relations with a man as something wonderful and that you build up your sons' sense of worth. They should perceive in what you say your love for their father and your appreciation of sexual expression in marriage.
Avoid presenting things solely in terms of fear—that is, in terms of mortal sin. The boys should understand that their manhood is a serious matter, and that is why they should avoid sin, but they should not be made to feel ashamed of their feelings or even of their possible falls into sin, especially solitary sin. They should be reminded that confession is always available if they fall, and they should never be questioned if they omit receiving Communion on any given occasion. This would be a good sign—namely, that they have developed a delicate conscience, and their mother should foster an atmosphere of freedom and privacy in these matters so that the boys do not associate sexual matters with constant fear and anxiety.