Leviticus 18:22 and 20:13
Leviticus 18:22 says, "You shall not lie with a male as with a woman; it is an abomination." Leviticus 20:13 says, "If a man lies with a male as with a woman, both of them have committed an abomination; they shall be put to death, their blood is upon them." While these verses do prohibit same sex intercourse between males, it is for ritualistic and cultural reasons rather than moral ones.
This section of Leviticus is often called the "Holiness Code". Along with some other things, it prohibits intercourse with a woman during her period, cross breeding of animals, sowing fields with two kinds of seed, wearing garments made of two different materials, marrying a divorced woman, tattoos, cursing your father or mother, eating meat with blood still in it, stealing, lying, adultery, witchcraft, and prostitution.
What do these things have in common and why were they prohibited? The Holiness Code is about purity. "Purity" because God required his people to be separate from the pagan culture they left behind (Egypt) and the pagan culture in which they lived (Canaan). It is meant to prevent the acts of pagan society altogether (where temple prostitution, male and female, heterosexual and homosexual, was common) because committing one might lead to another, like a domino effect. However, the biggest idea behind these verses is that temple prostitution is a sin. You might argue that it doesn't mention heterosexual intercourse as immoral and hence I'm dead wrong, but that's because women wouldn't be present at the temple near or during rituals in the first place (I'll get to that below). Many scholars believe that these verses are irrelevant outside of purity rituals for worship. Since the condemnation is ritualistic, and not a moral prohibition, it's not even applicable to anyone today. (Except maybe the part about temple prostitution in general...)
Notice women haven't been mentioned much up to this point? You'd think if same sex intercourse were being called immoral they'd be mentioned as well. But they aren't. Things don't add up if it's a condemnation of homosexual behavior in all of society. It only makes sense if the verses are condemning it in a ritualistic sense, since women were considered inferior and didn't take part in the rituals, so purity was irrelevant. This also makes sense because women are included when it comes to prostitution and adultery.