Musar על קידושין 42:21

"The Torah permitted the marriage with the אשת יפת תואר only in order to help the soldier in question cope with his evil urge." There are many things which have been permitted: Not because they are objectively permissible, i.e. desirable for us to enjoy, but because, were the Torah to forbid these things altogether, we might well become guilty of far greater transgressions. For example: it is better to eat an animal which has first been slaughtered though the blade of the knife may have been nicked, than to eat an animal which has died without the benefit of ritual slaughter altogether. Anyone who has some common sense will appreciate that just because the Torah appears to have made a concession to the fact that we are not angels and suffer from an evil urge, this is no reason to avail ourselves of any or all of these "concessions" to our moral weakness. The latter principle is known as קדש עצמך במותר לך, "demonstrate your attempt to sanctify yourself by abstaining from matter which is technically permissible."