Quote Originally posted by Soyeong View Post
A sound argument is one that is valid with true premises.
Yeah, I had to correct that same mistake when MaxVel pointed it out to me. It comes from the fallacy argumentum sin rubiacea, but I've had coffee and have corrected the fallacy.

Again, the assumptions he makes when establishing the foundation for natural law have nothing to do with the tenets of Christianity.
Utterly false, because his definition of nature (from whence he defined his interpretation of natural law) was his understanding of nature before the Fall (an explicitly Christian tenet). Without that definition, which he derived from Christian principles of what the Fall entailed, he could not have reached the same conclusions.

Aquinas' natural law is steeped in Christian doctrine. Had it not been, Aquinas' work would not have been permitted to be published.

If love was all that was necessary for marriage, then we would see a past of any two people in love getting married, which is not the case.
You are mixing cases: love (in our modern culture, we normally think of romantic love, but such need not be the case--there are other forms of love that will work) is the only positive requirement for marriage ... but there may be other negative requirements (one party is already married, one or both parties choose not to marry for some other reason, whatever). The absence or presence of these negative requirements is not relevant if the positive requirement is not met.

Contrawise, if procreation was the purpose of marriage, heterosexual couples unable to have children would be forbidden marriage. Indeed, if I remember correctly, Aquinas argued for precisely that (if he did not, other Catholics have, but that requirement has been relaxed of late).