The Church's teaching on matters sexual is a subject of much concern; ironically, far more outside the Church, than in the Church herself. No, I'm not saying it's not important to the Church, because it is; but this fact itself is often mocked: "why does the Church care so much about sex?" One answer would be, because human beings care so much about it! And, unfortunately, we handle it in ways that occasions a great deal of human suffering.
But the Church's teaching on matters sexual gets even more attention outside the Church, which is really a compliment: if the Church were as "irrelevant" as we're supposed to be, why spend so much energy analyzing, insulting, dismissing and mocking Catholic teaching?
It is funny to me how people end up saying rather silly things in discussing this, such as, "What's the big deal?" -- as if society is toodling along, paying very little attention to sex, and here comes lecherous Mother Church saying, "you wanna talk about sex?"
Put it another way--you don't need to have ever heard of the Church to know that sex is a big deal. Nor that it can occasion great human suffering. So why the surprise that the Church thinks its fraught with weighty moral considerations?
Nor do you need the Church to tell you that sexuality goes right to the core of human identity. So why should it surprise anyone to learn the Church believes that choices and habits in matters sexual are fundamental to our final destiny, either for good or ill? No, how I groom my hair isn't terribly significant for eternity; but then, perhaps that's because my hair is rather peripheral to my human identity.
Not so sexuality. Here's a thought experiment: think of a human being. Any human being, either someone who has lived, or is living, or a fictional human being, such as a character in a story. Picture that person, fill in the portrait, the image of that person.
Now, were we in a room, I'd ask: if your person was male, raise your hands. OK, if your person was female, raise your hands. And I feel very confident no one would remain who did not raise his or her hand.
Put it another way -- what would a truly "sexless" human being be? Not a man, not a woman. Even children have sexual identity. (Yes, sometimes this is messed up, either physically or psychologically. And its both very disturbing for the subject, as well as for others. Eventually does not that person have to opt for one or the other? Even someone pretending to be a man or woman, assumes a sexual identity for the sake of interacting with others, and perhaps even oneself.)
So, yeah, "sex is a big deal." And therefore, it is big deal morally.
It doesn't take much thought to realize that the one thing that would decisively discredit the Church would be if she did as her critics insist she ought, and simply shrug her shoulders about sex and said, "oh well, not a big deal."