I want to write (as much as I ever want to write, anyway), but unlike Darwin I am not gifted with rapid loquacity and I Have No Time. So here, a few quick links on Topic A.
Over at the National Catholic Register blog, Simcha Fisher posts on the sometimes ugly presentation of abstinence-based sex education, and on the right ways to talk to teens and pre-teens about chastity. In the second post, a commenter recommends a program for tweens called Purely You, which we are considering using with our now-ten-year-old.
Brandon posts a selection from Aquinas's commentary on 1 Corinthians. I've been meditating on this for a few days.
Hence it should be noted that the conjugal act is sometimes meritorious and without any mortal or venial sin, as when it is directed to the good of procreation and education of a child for the worship of God; for then it is an act of religion; or when it is performed for the sake of rendering the debt, it is an act of justice. But every virtuous act is meritorious, if it is performed with charity. But sometimes it is accompanied with venial sin, namely, when one is excited to the matrimonial act by concupiscence, which nevertheless stays within the limits of the marriage, namely, that he is content with his wife only. But sometimes it is performed with mortal sin, as when concupiscence is carried beyond the limits of the marriage; for example, when the husband approaches the wife with the idea that he would just as gladly or more gladly approach another woman. In the first way, therefore, the act of marriage requires no concession; in the second way it obtains a concession, inasmuch as someone consenting to concupiscence toward the wife is not guilty of mortal sin; in the third way there is absolutely no concession.
I'm not Aquinas, but I'm going to go out on a limb and guess that when he says "concupiscence", what he means is not arousal, which is a natural effect of the attraction between man and woman, but the lustful thoughts and fantasies and actions that can become entwined with sex, even in marriage. Note that he doesn't even touch on the quality of the sex the couple is having, because he is concerned with the motive and intent. Wild passionate sex in a marriage can be not just good, but meritorious if it's performed with charity; it can involve venial sin if it strays into concupiscence; it can be a mortal sin if it's mere satisfaction of lust. Bland and unexciting sex can be not just good, but meritorious it it's performed with charity; it can involve venial sin if it strays into concupiscence; it can be a mortal sin if it's mere satisfaction of lust. We don't always get to control our bodies. Couples meet at different temperatures sometimes, due to hormones or stress or illness or weariness or past experience. Bad sex is sex that involves mental or physical sin, not sex that isn't mind-blowingly fantastic. That said, spouses have an obligation to total self-giving, whether that's the gift of honesty about one's physical readiness ("I really am exhausted tonight") or the gift of surrendering oneself to be pleasured by the other.