if homosexuality is a sin, why isn't abstinence?

The question seems a bit problematic from the very beginning. If doing something is wrong, why isn't doing nothing also wrong? Or why isn't "not doing it" wrong? Just because God forbids one thing does not mean He mandates another.

The answer to the question really lies in why anything at all is wrong. There is a moral law, and it is not subjective, but objective. The Moral Lawgiver defines what is right and wrong. Sin is anything that is contrary to His purposes or plan.

The Bible states that God created one man and one woman for one marriage at the very beginning. That is the precedent that He set. He affirms later that this was, indeed, His plan and purpose, and that h*m*sexuality in particular is contrary to that.

We can also observe a specific design inherent in men and women. They were designed for each other, but it is fairly obvious that women were not designed for women, nor were men designed for men.

Adultery is also contrary to the pattern and precedent and purpose in marriage, and God identifies that as sin as well.

So, h*m*sexuality is a sin, not just because it cannot produce offspring and heterosexual relationships can. Homosexuality is a sin because it is contrary to God's specific design in marriage for one man and one woman, His purpose and plan for sexual fulfillment, and His establishment of the family.

Abstinence, on the other hand, is actually required until a person enters into marriage, so it certainly is not wrong.

I think that the easiest answer is that h*m*sexuality is "unnatural" (cf. Romans 1). Now, before someone tries to correct me and say, "it happens in nature all the time," I am not talking about "nature" in the same sense you might turn on the Discovery Channel and watch flying shark vs flying crocodile. After all, rape is something which is found in a number of different species and I would also call that "unnatural". No, "nature" in the Romans 1 sense has to do with how man and woman were created, how they were "from the beginning" (cf. Matthew 19).

When God created Adam and Eve, he created a mystical pairing whereby both parties partake in a supernatural act of self-gift (Paul calls it a "profound mystery" in in Ephesians 5). It is a pairing which is so profound that their bodies are actually designed to fit together and complete each other — so profound that the two become one flesh (Genesis 2). Not only that, but one of the primary results of this act is the creation of life (John Paul II basically said that sex is where we are the most like God in that we freely choose to create life).*

Homosexual acts are unable to properly demonstrate this pairing. While there may be love (and perhaps profound love, in a sense) between two individuals, if they are the same gender they are not compatible. It is not in their combined natures to unify that way.

I suppose someone might try to say, "So, it isn't in their nature, why is that wrong?" Well, is it in your forehead's nature to pound nails into walls? If you try, you will very quickly find that acting against your forehead's nature will cause you not a little bit of pain. Similarly, using any part of your body in a way which deviates from the original intent will prove damaging, even if that may not be the most obvious at the time.

^{* I think it is important to note that while it is one of the primary results of intercourse, children are not the point of sex. Using sex exclusively for the purpose of having children and divorced from the unitive and self-gift aspects is still placing the other as a means to an end.}