I'm going to be dissin' on Kant. If you loathe that sort of thing, maybe you'll enjoy reviewing the results of last year's poll by Brian Leiter, according to which Kant is the third most important philosopher of all time -- which should remind you that Kant's reputation is plenty safe from the likes of me.
According to Kant in The Metaphysics of Morals (not to be confused with his Groundwork for the Metaphysics of Morals):
(1.) Wives, servants, and children are possessed in a way akin to our possession of objects. If they flee, they must be returned to the owner if he demands them, without regard for the cause that led them to flee. (See esp. pages 278, 282-284 [original pagination], Gregor trans.) Kant does acknowledge that the owner is not permitted to treat these people as mere objects to "use up", but this appears to have no bearing on the owner's right to demand their return. Evidently, if such an owned person flees to us from an abusive master, we may admonish the master for behaving badly while we return what is rightly his.
(2.) Homosexuality is an "unmentionable vice" so wrong that "there are no limitations whatsoever that can save [it] from being repudiated completely" (p. 277).
(3.) Masturbation is in some ways a worse vice than the horror of murdering oneself, and "debases [the masturbator] below the beasts". Kant writes:
But it is not so easy to produce a rational proof that unnatural, and even merely unpurposive, use of one's sexual attribute is inadmissible as being a violation of duty to oneself (and indeed, as far as its unnatural use is concerned, a violation in the highest degree). The ground of proof is, indeed, that by it a man surrenders his personality (throwing it away), since he uses himself as a means to satisfy an animal impulse. But this does not explain the high degree of violation of the humanity in one's own person by such a vice in its unnaturalness, which seems in terms of its form (the disposition it involves) to exceed even murdering oneself. It consists, then, in this: That a man who defiantly casts off life as a burden is at least not making a feeble surrender to animal impulse in throwing himself away (p. 425).(If masturbation caused a permanent reduction to sub-human levels of intelligence, this argument might make some sense, but as far as I'm aware, that consequence is rare.)
(4.) On killing bastards:
A child that comes into the world apart from marriage is born outside the law (for the law is marriage) and therefore outside the protection of the law. It has, as it were, stolen into the commonwealth (like contraband merchandise), so that the commonwealth can ignore its existence (since it rightly should not have come to exist in this way), and can therefore also ignore its annihilation (p. 336).(5.) On organ donation:
To deprive oneself of an integral part or organ (to maim oneself) -- for example, to give away or sell a tooth to be transplanted into another's mouth... are ways of partially murdering oneself... cutting one's hair in order to sell it is not altogether free from blame.(6.) Servants and women "lack civil personality and their existence is, as it were, only inherence" and thus should not be permitted to vote or take an active role in the affairs of state (p. 314-315).
(7.) Under no circumstances is it right to resist the legislative head of state or to rebel on the pretext that the ruler has abused his authority (p. 319-320). Of course, the ruler is supposed to treat people well -- but (as with wives and servants under abusive masters) there appears to be no legitimate means of escape if he does not.
These views are all, I hope you will agree, odious -- even if there are some good things too in The Metaphysics of Morals (e.g., Kant condemns slavery on p. 329 -- although that was hardly a radical position for a European at the time). But why bring out these aspects of Kant? Shouldn't we expect him to be a creature of his time, an imperfect discoverer of moral truths, someone prone to lapses as are we all?
I mention these aspects of Kant to draw two lessons:
First, from our cultural distance, it is evident that Kant's arguments against masturbation, for the return of wives to abusive husbands, etc., are gobbledy-gook. This should make us suspicious that there might be other parts of Kant, too, that are gobbledy-gook, for example, the stuff that transparently reads like gobbledy-gook, such as the transcendental deduction, and such as his claims that his various obviously non-equivalent formulations of the fundamental principle of morality are in fact "so many formulations of precisely the same law" (Groundwork, 4:436, Zweig trans.). I read Kant as a master at promising philosophers what they want and then effusing a haze of words with glimmers enough of hope that readers can convince themselves that there is something profound underneath.
Second, Kant's philosophical moral reasoning appears mainly to have confirmed his prejudices and the ideas inherited from his culture. We should be nervous about expecting more from the philosophical moral reasoning of people less philosophically capable than Kant.