Sex is a frequent topic of debate where religion is concerned and as a result religion has come to be associated with a sex-negative attitude. While there are many religions that do have what some would consider an oppressive view on sex, Judaism is not among them. In fact, Jewish sexual laws are rather progressive even by today's standards and were incredibly forward-thinking for the time in which they were established. Sex in Judaism still revolves around the concept of marriage, but there are several reasons for that beyond the inherent conservatism of that perspective. Mostly, Judaic sex laws aim to create as much harmony and equality between couples as possible.
To understand the Jewish perspective of sex one must first understand the concepts of Yetzer Tov and Yetzer Ra. These terms translate, respectively, as "impulse of good" and "impulse of bad". Desires that fall into one category or the other are not considered inherently good or bad, merely compelled either by selfishness or the good of others. Desires that originate in natural urges, such as hunger, thirst and the desire for sex, are under Yetzer Ra because they inherently involve the immediate urges of the individual. These desires can be balanced by appealing to Yetzer Tov, saving them from being selfish and potentially harmful. Just as it would be sinful to hoard food all for one's self, it is sinful to engage in sex with no regard for the desires of one's partner.
This is why sex in Judaism is limited to marriage. A Jewish marriage is, by all accounts, a legal partnership. It is based on a signed, witnessed agreement. As such, a married couple have certain obligations to the health and happiness of one another, including sexual gratification. Both in the Torah and later expanded in the Talmud there are rules about how often a married couple is obligated to have sex with one another independent of any attempt to procreate. The majority of the burden of this obligation is traditionally placed on the husband. He is actually bound by law to meet the sexual needs of his wife as she expresses them. Furthermore, he is strictly forbidden to ever force his wife to engage in any kind of sexual behavior.
By the same token, both parties in a Jewish marriage are not permitted to use sex or the lack thereof as a form of manipulation. It is a sin to withhold sex from one's partner or to compel it as currency or punishment. The Talmudic sages even created a rough framework for how frequently a man is obligated to have sex with his wife based on the travel and freedom related to his occupation.
Loving sex between a married couple in Judaism is considered a great mitzvah because it strengthens the bond between the couple and celebrates one of the pleasures of being alive. It is especially encouraged on the sabbath and other holidays. This is because sex is viewed as an act of creation, even when it doesn't or can't produce a child. The creative act is instead the creation of joy, love and cooperation.