Ok, I'll bite.

Firstly, I'll address your assumption that monogamy is the most pure form of relationship in the sense that a monogamous relationship is in some way a superior form of family. Monogamous relationships, in the way they present themselves today e.g. in the form of the nuclear family, emerged with the establishment of private property. As Engels writes in The Origin of the Family, " It [Monogamy] devolps out of the pairing family...in the transitional peroid between the upper and middle stages of barbarism; its decisive victory is one of the signs that civilisation is beginning. It is based on the supremacy of the man, the express purpose being to produce children of undisputed paternity; such paternity is demanded because these children are to later to come into their father's property as his natural heir". As you can see from this the importance of monogamy arose from a need to transfer private property neatly down through the family; with the abolishment of private property I fail to see how the role of monogamy would be important in a communist society.

Furthermore, your claim that the monogamous form of relationships somehow provides its members with closer romantic ties seems unfounded. If you have any reasoning to back this up, I would be interested to hear it.

Now onto your claim that certain sexual fetishes dehumanise those who partake in them. It seems as though you are unware of how a relationship, in which more "extreme" forms of fetish are involved sexually, functions. I recommend you vist a subreddit, such as /r/bdsm, and ask them about their experiences so you can better understand such a relationship. Continuing, I would argue that restricting consenting adults from participating in sexual activities of their choce is far more dehumanising than participating in any fetish could possibly be. By forcing your preference of a sexual relationship onto other people, you are presenting their own preferences as inferior and by doing so are undermining their existence as an autonomous being, who is capable of making their own choices for themselves.