May a Kohen become tamei to his wife min haTorah

This piece will show how peshuto shel mikra is the exact opposite of how Chazal read it, with Rambam codifying according to the former

Rashbam explains the pasuk in Emor (Vayikra 21:4) לֹ֥א יִטַּמָּ֖א בַּ֣עַל בְּעַמָּ֑יו לְהֵ֖חַלּֽוֹ׃ to mean that a kohen is not allowed to become tamei to his wife שום בעל בעם כהנים לא יטמא לאשתו. להחלו – שהרי מתחלל מכהונתו and that according to Chazal it means precisely the opposite ולפי דברי חכמים (ספרא ויקרא כ״א:ד׳, בבלי יבמות כ״ב:): לא {יטמא} לאשתו פסולה ומחוללת, אבל מטמא לאשתו כשירה.

Two pesukim prior (ibid:2) the Torah states כִּ֚י אִם־לִשְׁאֵר֔וֹ הַקָּרֹ֖ב אֵלָ֑יו לְאִמּ֣וֹ וּלְאָבִ֔יו וְלִבְנ֥וֹ וּלְבִתּ֖וֹ וּלְאָחִֽיו׃ which Onkelos renders as אֱלָהֵין לְקָרִיבֵיהּ דְּקָרִיב לֵיהּ לְאִמֵּיהּ וְלַאֲבוּהִי וְלִבְרֵיהּ וְלִבְרַתֵּיהּ וּלְאֲחוּהִי whereas acc to Chazal the meaning is, as Rashi brings, אין שארו אלא אשתו, (see as well targum yerushalmi אלהין לאינתתא דקריבא לגופיה לאימיה ולאבוי ולבריה ולברתיה ולאחוי).

Essentially what we have here is peshuto shel mikra not making an allowance for a kohen to become tamei to his wife with Chazal allowing it (with the caveat that the marriage is permissible) and interpreting the pesukim in a way that supports them.

In the parasha of nachalos (bamidbar 27:11) the pasuk reads וְאִם־אֵ֣ין אַחִים֮ לְאָבִיו֒ וּנְתַתֶּ֣ם אֶת־נַחֲלָת֗וֹ לִשְׁאֵר֞וֹ הַקָּרֹ֥ב אֵלָ֛יו מִמִּשְׁפַּחְתּ֖וֹ וְיָרַ֣שׁ אֹתָ֑הּ וְֽהָ֨יְתָ֜ה לִבְנֵ֤י יִשְׂרָאֵל֙ לְחֻקַּ֣ת מִשְׁפָּ֔ט כַּאֲשֶׁ֛ר צִוָּ֥ה יְהֹוָ֖ה אֶת־מֹשֶֽׁה׃ with pashta dikrah making no mention of a husband inheriting his wife with Chazal again saying that he does and reinterpreting the pasuk as such (bt bb 111b) שארו זו אשתו, מלמד שהבעל יורש את אשתו

Interestingly both these laws center on the talmudic definition of the same word—שאר, with Chazal saying that in both cases (tumas kohen and nachalos) it refers to a wife

Theres a disagreement (bt bk 42b) if the husband inherits biblically or rabbinically

Rambam (eg mt nachalos 1:8) comes down on the side of it being rabbinic

As it relates to tumah Rambam writes (eivel 2:7) אשתו של כהן מתטמא לה על כרחו, ואינו מתטמא לה אלא מדברי סופרים. עשו אותה כמת מצוה, כיון שאין לה יורש אלא הוא, לא תמצא מי שיתעסק בה אלא הוא. ואינו מתטמא אלא לנשואה בלבד, אבל הארוסה לא יתטמא לה

The commentaries point out that the Talmud only applies the status of meis mitzvah to a wife that is married rabbinically, with regard to a biblically recognized union the Talmud relies on the above mentioned exegesis

It seems reasonable to assert that Rambam holds that as peshuto shel mikrah does not support these laws they are both rabbinic (see shm shoresh 2), particularly with there being talmudic precedent for that assertion by yerusha and that by tumah he only need amplify the meis mitzvah application which is already mentioned in the gmara 

Not only that, but becoming tamei to a wife is dependent upon inheriting her. Being that Chazal gave a woman's inheritance to her husband, other relatives that should have inherited will balk at burying her, they therefore gave her the status of a meis mitzvah vis a vis her kohen husband, who does inherit her, and obligated him in getting involved with the burial along with all that it entails including contracting tumah from her.