Well, for us to have meaningful communication, we must define our terms. We must know what we are trying to say to one another.
Although natural law can also be thought of as the sum of the physical laws of natural matter, the Catholic Church means by the term Natural Law,
that law which relates specifically to the nature of Man.
Man’s nature in this physical world is a uniquely rational nature, since only Man amongst all the animals has the ability to reason.
Here is an article on Natural Law in the Catholic Encyclopedia:
newadvent.org/cathen/09076a.htm
…and another from good ol’ Jimmy Akin of Catholic Answers fame:
catholic.com/thisrock/2000/0010bt.asp
Since this is a Catholic forum, and we are presumably striving to understand issues from a Catholic perspective, perhaps it is better and less confusing to substitute the phrase “The Law of Reason” when we speak about Natural Law and what the Catholic Church means by it. I recently heard a bishop define Natural Law this way in his homily, which was mandated to be listened to on tape throughout his diocese, so this is not just my own idea.
With regard to sexual activity, “The Law of Reason” tells us - we can see for ourselves - that the genitals of the male fit together with the genitals of the female to fulfill the purposes of sexual union. No homosexual activity can possibly do this. Homosexual activity - i’m sorry to say it this way and you are sorry to read it this way but as a homosexual myself I say to you let us look honestly at what we’re actually talking about - homosexual activity is a misuse of sex. It is a perversion of sex.
Once we have defined our terms - i.e. we understand what the Church is actually trying to tell us - I think it is pretty hard to disagree with the Catholic Church’s teaching that homosexual activity goes against Natural Law. The only way to do it is to misconstrue what is being meant by “Natural Law”, thereby sabotaging communication.