悪場所といえば芝居と廓ということになろう means "When we say 悪場所, it would usually refer to theater districts and red-light districts." 芝居 refers to 芝居町, which was a big place of amusement in those days (see 江戸三座). 廓 basically means "district" but it could refer to 遊郭 on its own. So they are talking about such special districts detached from the ordinary life.
This sentence is very hard to interpret. This looks like a poem rather than an academic analysis, especially to someone who is not familiar with the theory of the author (廣末). I couldn't even find the definition of 一同性; maybe it's synonymous for 普遍性 "universality"? Anyway, you can read the summary of what the author thinks about 悪場所 in this blog article.
The author regards our ordinary life outside 悪場所 as 聖なるもの ("something sacred"), and he is contrasting it with the life in 悪場所. Ordinary people believe 悪場所 is a special place, but once you're in an 悪場所 and see the life outside, ordinary life will start to shine as something special and sacred (対岸のそこに存在する = "the sacred thing is always there at the other side of the river"). This is a guess, but 聖は悪の影を宿す (literally "sacredness will have a shade of evilness") may be saying 外の世界 (=聖) and 悪場所 (=悪) are just two sides of a coin. Same sex marriage does not seem to be related to this.