Thanks for your question Jodi.
I am assuming that what you are asking is why the physical act of sex between persons of the same sex is considered sin. If I am wrong in this assumption, I apologize.
To address that particular topic though, the simple answer is that our God, who created us and knows everything about our makeup, says that it is sin for a man to lie with another man the way that males lie with females. The same holds true for two women. The Bible says it is against nature (or contrary to what is natural).
Romans 1:26 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.
It also calls such affections vile or passions of dishonor. We have to remember that the prophecy (of the scriptures) came not in old time by the "will of man: but holy men of God spake as they were moved by the Holy Ghost"
2 Peter 1:21 For the prophecy came not in old time by the will of man: but holy men of God spake as they were moved by the Holy Ghost.
God had things written in His Word that He wants us to know.
People experience many emotions in our lives. Some of those emotions we know we cannot act upon because the effect of these actions would be harmful. In some situations we may not see the harm in certain behavior right away. That doesn't mean its not there. That is one of the reasons God gave us His Word. We have a standard to refer to when we have questions.
Always remember that God loves you and He can help you deal with anything.
-- Tim Plear