As it says in the book of Genesis, God made humans in his own image and "male and female he created them." So, we should ask what is God's divine plan and purpose for our sexuality. Of course, the most obvious purpose is procreation. Humans are made male and female in order to conceive and bear children, and thus to "be fruitful and multiply." Clearly, this is the reason that animals are also male and female. But, there is another purpose for sexuality in humans that is not the case for animals. For God's gift of human sexuality is also intended to bond a man and a woman together emotionally and help build their love for each other through their mutual self-giving. After all, it's not without reason that sexual intercourse is commonly called "making love."
Unfortunately, our culture leads many people to believe that sex is for recreation. Of course, sex is pleasurable. But, that is simply God's way of making sure we humans don't ignore the need to "be fruitful and multiply." In the same way, our hunger is what motivates us to eat food. If the purpose of eating was pleasure, then since it gives us pleasure to eat, let's purge and eat some more. The reason we shouldn't is that the purpose of eating isn't pleasure, but nutrition. We don't eat just to satisfy our bodily desires (hunger). In the same way, sex is about much more than satisfying our bodily desires.
Human sexuality has two distinct but related purposes. The first purpose is life-making. The second is love-making. The Catholic Church calls these the procreative and unitive purposes. This means that sexual intercourse is morally wrong (sinful) whenever it's done outside of a relationship that's open to both of these divine purposes. For example, prostitution is morally wrong because it's neither for love nor for children (it is also wrong because a person's human dignity is reduced to being an instrument of mere sexual pleasure). Similarly, homosexual acts are morally disordered because they are not in accord with these two divine purposes, since it is impossible for children to be conceived in such a relationship.
In fact, the Catechism of the Catholic Church (CCC #2351) clearly states, "Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes." Christians who accept God's plan for our sexuality are freed from slavery to their passions. They are less likely to behave like animals in heat! Rather, they're able to be truly human.
It is also interesting to note how differently men and women often view sex. Even though there are dangers in generalities, the following saying is quite enlightening. It's been said that, "Guys play at love to get sex, while girls play at sex to get love." Of course, this is not always the case, especially within a committed, long-term relationship. However, this saying reflects the underlying tendencies often found in the dating scene. This is especially apparent when it's said backwards. Backwards, the saying would be, "Guys play at sex to get love, while girls play at love to get sex." There is usually a negative reaction from both sexes when this saying is said backwards. Girls instinctively believe it's "gross" or "sick" to "play at love to get sex." But, guys often don't see anything unusual in that.
Therefore, it's important for everyone (especially young women) to be aware of their own possible motives, as well as those of the opposite sex. Just because a girl is feeling deeply "in love" and may want to share her all, that may not be where the guy is coming from at all. Sadly, all too often he could be "playing at love" merely to get sexual gratification.
Another part of God's plan is also apparent in the above saying. God wants to ensure the continuity of the human race. Therefore, God seems to have instilled in men a generally stronger desire for sex. However, God also wants children to be raised in a loving and nurturing environment. That's why God seems to have instilled in women a generally stronger desire for romantic love and an instinctive willingness to want sex only with a man she loves.
Imagine what would happen if God had made men and women the same. If men were like women, children would rarely be conceived since most women can more easily live without sex. And if women were naturally willing to have sex with anyone, children would be far less likely to be raised in a nurturing environment with loving parents present. So, we see that the innate but different tendencies of both sexes are needed in order to fulfill God's plan for the human race. Men and women are made to complement and complete each other.
In part to avoid problems associated with misusing God's gift of our sexuality, the Church has always taught that sexual intercourse is morally permissible only within a committed and lifelong relationship of love; that is, within marriage. Only within heterosexual marriage is sex "noble and honorable" (CCC #2362). Marriage is where both divine purposes (love-making and life-making) of the gift of our human sexuality can be truly fulfilled.