First, there is the logic. Numerous male bloggers have demonstrated why the male equivalent of "slut" is "stud". It is relatively difficult for a man to be sexually successful with women. It is relatively easy for women to be sexually successful with men. It is all about the degree of difficulty involved, which is precisely why promiscuous homosexual men are not begrudgingly respected the same way promiscuous normal men are. As the apt analogy has it, the key that can open a thousand different locks is a master key, whereas the lock that can be opened by a thousand different keys is a defective lock. There is no double standard, there are two different standards, distinguished by the varying degrees of difficulty.
Of course, logic is dialectic and the female attempt to attack "male sluts" and "manwhores" is observably pure rhetoric. So we know, per Aristotle, that the logic will be insufficient in addressing the issue and it is necessary to find another, more rhetorical means of convincing those who have adopted such terminology in a futile attempt to shame men out of sexual desire that their efforts are as misguided as they are ignorant.
Therefore, let's look at the definition of the word:
1. an immoral or dissolute woman; prostitute.The man reason that one cannot reasonably use the term "slut" for a man is that its primary meaning is not related to its immoral or promiscuous aspect, but rather its intrinsically female aspect. To call a man a slut is not to label him promiscuous, but rather, to label him a specific type of woman. Hence Chaucer's choice of the term "sluttish" to refer to a man in the 14th century rather than "slut". This has been the case for more than 600 years, which is very nearly the time in which the English language has existed. While it is true that languages change over time, no amount of defiantly declaring that black is white or war is peace will actually change the RGB values or cause the armies to vanish.
2. Obsolete . a dirty, slovenly woman.
Origin: 1375–1425; late Middle English slutte; compare dial. slut mud, Norwegian (dial.) slutr sleet, impure liquid
Word Story: Slut first appeared in the written language in 1402, according to the Oxford English Dictionary , that great repository of language information. At that time, slut meant roughly what one sense of slattern means today: a slovenly, untidy woman or girl. It also apparently meant “kitchen maid” (”She is a cheerful slut who keeps the pots scrubbed and the fires hot.”). By the end of the 15th century the sense “a woman given to immoral or improper conduct” had come into use, and it is the only meaning in use today. Interestingly, the same second meaning, a promiscuous woman, developed for the term slattern.
A slut is, and has always been, a woman. It can mean an untidy woman or it can mean a promiscuous woman. But the one thing it cannot mean is a man; at most a man can be described as "sluttish", which is a more specific adjective than the similar, but more general term, "effeminate". To attempt to argue otherwise is not only ignorant and illogical, but uneducated. One might as reasonably attempt to claim that the words "mother" and "girlfriend" can be applied to a man.