Five times in this Parsha the Torah uses the term To'eiva, an abomination. These are:
משקלות False Weights and Measures
כלי גבר Wearing the clothing of the other gender in order to secretly pass among them
אתנן זונה Bringing a Korban of an animal given in payment for prohibited sexual relations.
מכיר כלב Bringing as a Korban an animal received in exchange for a dog.
מחזיר גרושתו משנשאת Remarrying a woman you had divorced who had been married in the interim.
Reb Moshe says that the תועבה aspect of false weights and measures and wearing the clothing of the other gender refers to these things done as a precursor to sin. The case of the false weights is not that he actually used them, but he just has them in the house, and he has no intention of using them. (Chinuch 602, רמבם ז גניבה ג- כל מי שמשהה בביתו או בחנותו מדה חסרה או משקל חסר עובר בלא תעשה שנאמר לא יהיה לך בכיסך וגו'. ואפילו לעשות המדה עביט של מימי רגלים אסור.) He doesn't even have a particularly big yetzer hara to use them. On the contrary! He will keep these false weights in his house to revile them, as the Rambam says, or, by not using them, he will be rewarded for resisting and overcoming his Yetzer Hara. It's a Big Mitzvah! But that is exactly what a to'eiva is- to put yourself into a position where sinning will be easier, to put yourself into a position such that later, when a moment of weakness and desire comes along, the means of doing the sin will be at hand. The case of Kli Gever means that he just enjoys wearing women's clothing or hanging around with women, but he has no intention of sinning. He tells himself that he will tantalize himself by the availability of sin and refrain, and he will be rewarded by the Ribono shel Olam-another Big Mitzvah! But inevitably the moment will come that he will have a yetzer hara, and because he has all the appurtenances and experience, sinning will be as easy as falling off a log. If you sin because you were seized by a yetzer hara, it's bad enough. Here, you didn't even have the yetzer hara. At that time, at the moment when you don't have a particular Yetzer Hara, what you ought to be doing is creating a home that is safe from temptation. So why are you exactly the opposite? Why are setting things up for that moment of weakness so that the sin will be easier? That is a to'eiva.
The to'eiva of the third and fourth are that the person wants to buy a clean conscience by doing a mitzvah. This is obvious in the case of an Esnan Zona. In the case of Mechir Kelev, you can either say like Reb Meir Simcha, that the essential issur hearkened back to the time that certain dogs were kept for the purpose of bestiality, or because in many places, having a dog meant that you didn't want poor people to bother you asking for money. Having done these ugly things, you feel bad. You tell yourself, I feel dirty, I feel ashamed. I need to cleanse myself, to heal my soul! What you ought to be doing is Teshuva, making yourself a better person. Instead, you bring the animal as a korban. It's a Big Mitzvah! All you're doing is buying a clean conscience. As someone once said, "Whenever I get the urge to exercise, I lie down until the feeling passes." This person doesn't just lie down; he lies down and eats donuts until the feeling passes. This perversion of a good impulse to do teshuva into a superficial sop to the Ribono shel Olam, this selfish delusion that you are a holy man while you remain a sheigitz, is a to'eiva.
The fifth, machzir gerushaso, is a person that likes the idea of spouse swapping. He wouldn't chalila transgress the issur of eishes ish! The very thought sickens him. But lemaisah has this tremendous desire for his neighbor's wife, as his neighbor does for his. So they divorce their wives, switch exes, and later they take them back. It's a Big Mitzvah! I am not like those rotten bums that have affairs with an Eishes Ish. Oh no, I am avoiding that terrible issur by finding a way to make it muttar! This person ought to be cauterizing his animalistic desires, but instead, he uses his knowledge of the Torah as a servant to his rotten desires. You know that what you want is disgusting, and you know that you would never do the issur. So why are you not addressing your yetzer hara??? Why, instead, are you looking for a way to kasher it??? That is a To'eiva.
All the To'eivos in the parsha involve people who twisting the ideas and the values of the Torah to convince themselves that everything they are doing is a fulfillment of the Torah, and by doing this, they blind themselves to their Rish'us. These people tell themselves they are showing discipline by avoiding sins, they are doing good things, they are fulfilling the dinim of the Torah. They find frummeh ways to telling themselves זכיתי לבי טהרתי מחטאתי.
The To'eiva is the abuse of Torah concepts in the furtherance of Rish'us. These people are perverting the pious impulse that every Jewish heart experiences by using it to quiet their conscience and fuel their Yetzer Hara. It would be better to follow Eliahu Hanavi's advice: אם ה' האלוהים לכו אחריו ואם הבעל לכו אחריו.
These To'eivos form a progression.
First, a person tells himself "I have no interest in sinning." But he brings the yetzer hara into his house, and sets himself up for the time when he will sin.
Second, a person sins, but instead of facing his failure and doing a proper teshuva, he brings the proceeds of his sin as a Korban, and thinks that this will cleanse him. This doesn't cleanse him, it makes him dirtier. This is That is a To'eiva.
Third, a person says "Oh, I would never do that terrible sin! But I have a tremendous desire for my neighbor's wife. What shall I do???" The answer at this point is, you realize that what you want is horrible, you know that you're a chazer for even thinking about it. So take a cold shower, eat some chasteberries, and stay away from your neighbor's wife. Don't talk to her on the phone, don't accidentally go shopping when she's likely to be out shopping, and don't invite them to the house on Shabbos. But he has a better answer, "I and my neighbor will divorce our wives, switch partners, and then take them back." That is a To'eiva.
Speaking of To'eivos fueled by degenerate piety, A B C D E F G