Basically: Being gay is punishable by death.
Historical background: Leviticus and its so-called holiness code had a long period of transmission and editing, probably reaching its final form in post-exilic Israel (c. 530s BCE.) The book comprises both the "Priestly code" and the "Holiness code," which both contain instructions for priests and Israel alike to lead pure lives in front of God. This code for sexual ethics in Leviticus 18 also includes prohibitions against incest, bestiality, and sex with a menstruating woman.
What more conservative readers might say: To be fair, many churches have abandoned a literal interpretation of this verse, as the same Levitical holiness code forbids eating pork and wearing clothing made from two different fibers. But in both Christian and Jewish tradition, these verses have been used historically as a blanket prohibition against homosexual behavior. Here's one evangelical commentary that argues that "ceremonial" laws, like the one prohibiting consumption of pork, are no longer valid, but that the "moral" laws, like the one prohibiting homosexual behavior, still are.
What more liberal readers might say: The prohibition of gay sex must be read in the historical context of the time. There was really no contemporary equivalent of a loving homosexual relationship like we have today, much ancient same-sex activity involved pederasty. In addition, male "seed" was prized as valuable for producing offspring, and any activity that "wasted" it was frowned upon. (See also the injunction against "onanism" in Genesis 38.)