This comment from me always stirs up such a sh*tstorm.
Lou, Yeah I never went along with the No MB thing for the very reason you cited--I'd be a lunatic. MrHP has accepted a portion of the responsibility of helping me to stay away from this temptation and has agreed to regular sex. Regular for us is 2-4 times per week, so while the urge still hits, it's managable. His stance might not be all that popular with the LD folks, I don't know, but it's his nonetheless.
Annette, Catholics believe that there are two purposes of ML to your spouse: Unitive and procreative. Unitive obviously is talking about uniting (in body and spirit) the two people. Closeness and intimacy, IOW. Procreative refers to the ability to create new life. Contraception places a barrier between the two people and therefore the sexual gift is incomplete. Again, this doesn't mean that you have to have 8 kids in 12 years; it merely means that if you want to space your children a certain way, that you abstain during fertile periods.
I have to say that I am not as up to date on the specific logic as, say, GGB may be since I am infertile. It doesn't apply to me, either way. I've taken the pill on and off throughout my life to regulate the unchecked estrogen that circulates in my system, but I don't know that I will ever take it again. I think I will look to other hormonal methods to regulate the estrogen that do not additionally contain elements that make the uterus inhospitable to a growing embryo. I would hate to think that I became pregnant, on my own!, and the pill ruined that experience for me, since I wouldn't be taking it to prevent a pregnancy anyway.
I hope that makes sense. I looked for a concise and clear article that explained it better than me, but you know religious documents..nothing concise about any of em, no matter what religion you are.