Homosexual marriage is wrong on two levels. First it's not a marriage. Second it's not a sexual act. The reason why it's not a marriage is already discussed above. The reason why it's not a sexual act is because it's contrary to the purpose (recall Aristotle's fourth cause, the final cause) of a sexual act. The main purpose of sexual act of the genitals is to produce offspring. Homosexuality use the sexual organs contrary to its purpose. An infertile heterosexual couple does not use their sexual organs contrary to its purpose because they remain open to the possibility of procreation.

So sexual organ are meant for [ie. it's final cause/purpose is] heterosexual sex, just like how the heart is meant for pumping blood, just like how the kidney is for cleansing blood. We know this. But if after knowing it we use our sexual organ for homosexual act, our heart for cleansing blood and our kidney for pumping blood then we are acting contrary to what we know is the organs purpose [.ie. final cause]. It's like knowing that 1+2=3 but deliberately say that it's 4. Such acts destroy reason. We are being unreasonable.

Wonderful explanation.

But I have two problems.

One, it seems there is a hierarchy of applicable actions that a hammer can be used for besides inserting and extracting nails. The applicable actions that destroy reason the least are things that are closer to its designed purpose and the applicable action that destroy reason the most are thing that are further from it designed purpose.

Using a hammer to crack open a piggy bank... probably pretty close to what the hammer was designed for but not precisely what it was designed for.
Using a hammer to knit a scarf for the winter... pretty far from what the hammer was designed for.

Second, with all that in mind it makes me think that using things contrary to their purpose is not such a bad thing. But, perhaps using a hammer to knit is an acknowledged deterioration of reason which could argued to be a sin, and when the deterioration of reason is born of misusing one of God's creations (human) the sin is magnified.

Does that make any kind of sense?

That's even better. I never thought about a hierarchy. Or maybe, as I had in mind, two opposite poles. On the most remote part of the pole right side you have "using hammer to hit the nail to the wall" (the very purpose of a hammer) and on the most remote part of the pole left side you have "using hammer to pull the nail off the wall" (very contrary to the purpose of hammer).

Well, it seems a hammer has two different parts that interact with nails. The hitting (pushing) and the extracting (pulling). In any case, wouldn't one be devoid of reason already when you go to try and knit with a hammer. It wouldn't necessarily decrease reason itself, but rather be a symptom of decreased reason...