Not to all 'Sunday School' on the topic, but obviously, whoever taught that was woefully uneducated in the Bible. Sex is not sinful, unless done outside of marriage (fornication/adultery/sodomy).
A bit of a simplification. In his first letter to the Corinthians, Paul portrayed sex as the less moral of the two options when compared to chastity, but acknowledged that marital sex was preferable to "burning." This premise colored much of Catholicism's historical viewpoint of sex and it wouldn't be unfair to say there were long periods where it was viewed as a necessary evil. Of course modern Catholics are likely to be much more optimistic, and part of what separated many protestants from early/mid Catholics was that some of the former had more positive interpretations of marital sex. Protestant writer John Milton, for example, portrays the sexual relationship between Adam and Eve in Paradise Lost to be both wholesome and beautiful.
I do largely agree with everything else you posted, though. The Fall and original sin really have very little to do with sexuality, and I would argue the mainstream perception of the terms is cognizant of that.
