As Yuuichi Tam has pointed out in a comment, you have misread 一回性 as 一同性. This error is compounded by your assumption that 一同性 has something to do with same-sex marriage. It does not. (In fact, as far as I know 一同性 isn't really a word, though of course 一同 is.) The meaning of 一回性 is difficult to convey in a single word or short phrase in English, but it refers to the nature or quality of an event that is unique and occurs only once, never to be repeated in the same way.
I haven't read Hirosue's work, but judging by the context it would appear that when he writes of the 一回性 of the sacred, he has in mind the temporary but often very dramatic suspension of ordinary mores and norms that occurred during religious events like the matsuri in premodern times. A somewhat analogous atmosphere of licensed misbehavior prevailed in theatre districts and pleasure quarters during the Edo period, with the crucial distinction that it was not fleeting, but permanent. In such spaces, behavior that was normally forbidden (or was permitted only for a short time within the carefully defined physical and social space of a religious event), was allowed to continue on an ongoing basis. That's what Hirosue is getting at when he writes 「本来、持続してはならないものが持続しており、そのために、恣意的に関係を結ぶことも可能になるような場が、悪場所であった」. (The arbitrarily formed relationships referenced here are presumably those between prostitutes and their clients, and between some kabuki actors and their admirers.)
In light of the above, we can translate the first sentence you asked about as follows:
In such spaces, the unique, never-to-be-repeated character of the sacred is negated. Moreover, the sacred never completely lapses into the quotidian profane, but instead always remains in existence in the other realm [i.e., the world outside the pleasure quarters]. Thus, the sacred contains the shadow of evil.
The assertion that the sacred contains the shadow of evil seems like something of a non sequitur; maybe the connection is elucidated in the original text. But the gist of the sentence as a whole seems pretty clear, if rather abstract: On the one hand, within these "bad places," the special nature of the sacred that obtains elsewhere -- its 一回性 -- is denied or subverted. But on the other hand, for those within these spaces, that which is sacred (or sacrality itself) never loses its sacredness and becomes ordinary, as it inevitably must elsewhere. Instead, it exists permanently in its ideal state, albeit at a remove.
As for your second question, I'd translate that sentence as "When we speak of 'bad places,' we are generally referring to theatre districts and pleasure quarters." If you aren't familiar with this usage of なる, I suspect you can find an explanation of it elsewhere on this site.