Lam writes:
Show us, directly from the bible, why it's a sin.
Lam, to my knowledge, the bible doesn't explain why it is a sin.
There is, though, the statement that homosexuality is contrary to God's design:
Romans 1:21-27:
21 Because that, when they knew God, they glorified him not as God, neither were thankful; but became vain in their imaginations, and their foolish heart was darkened.
22 Professing themselves to be wise, they became fools,
23 And changed the glory of the uncorruptible God into an image made like to corruptible man, and to birds, and fourfooted beasts, and creeping things.
24 Wherefore God also gave them up to uncleanness through the lusts of their own hearts, to dishonour their own bodies between themselves:
25 Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen.
26 For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature:
27 And likewise also the men, leaving the natural use of the woman, burned in their lust one toward another; men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet.
[Emphasis mine.]
It's hard to argue against the intended design (assuming such) seeing as how the male and female anatomy are complementary with regards to sexual intercourse.