You'll find a huge range of responses, ranging from "I am who I am and fuck anyone who doesn't like it" to "I would never, ever dare breathe the slightest whisper to suggest that I did not conform to social norms in every way." And you'll find just as many reasons for these attitudes.
On the "I am completely closeted" side of the equation, many of the reasons center around a few simple ideas: fear of tangible loss ("If my ex found out I'm poly, he might try to take custody of my child," "If my boss found out I'm gay, I'd be fired"), fear of being judged ("My parents would never approve," "My friends would think I'm a slut if they found out I have two lovers"), fear of emotional loss ("My friends would not like me any more if they knew I was bisexual," "My mother would disown me if she knew I have two husbands"), and that sort of thing.
On the flip side, you'll find the same arguments often trotted out to counter these ideas ("If more people asserted their rights to child custody who were openly pagan/gay/whatever, the social structures and stereotypes that allow such people to be cast as unfit parents would fall," "if someone loves you and then, after learning the truth about who you are as a person, withdraws that love, then that person never loved you to begin with," "you can not love someone you do not know," "if your friends only like you as long as you project a false image of yourself to protect their own prejudices, you need a better class of friend.").
You'll also see arguments in favor of remaining closeted based on the specific situation of the person in the closet ("I'm in the military," "I work for a church that condemns homosexuality") and arguments that rebut those arguments ("You had a choice about joining the military," "If you're gay and working for an organization that promotes disenfranchisement of gays, you're shooting yourself in the foot and working against your own interests.") And 'round and 'round it goes.
Now, I'm firmly on the side of "I am who I am and fuck anyone who doesn't like it." I do not see the advantage of pretending to be someone I'm not, nor see any compelling reason to protect others from the emotional consequences of their own prejudices. But that's not actually what I'm here to talk about. I'm here to talk about a more subtle, and potentially more insidious, problem that can arise frm remaining tightly closeted, especially in polyamorous relationships--with acknowledgment to feorlen for starting me thinking along this path.
Doing the abuser's work: the closet as exile
One of the first, earliest hallmarks of a classic abusive relationship, counsellors and mental health professionals will say, is a relationship in which one person seeks to isolate his or her partner, cutting the victim off from friends and family, controlling who the victim may socialize with, and seeking to limit the victim's contact with other human beings.
This is a useful tool for an abuser. A situation where one or more people are denied access to contact with other people creates an environment where a person may not notice destructive, unhealthy aspects of the relationship. Often, a relationship's dysfunctions are invisible from the inside; without someone from the outside to say "Whoa, dude, that's totally fucked up!" it becomes easy to be blinded to even the most blatantly destructive, unhealthy things in a relationship.
Even outside the context of abuse, the presence of extra, uninvolved pairs of eyes is often useful for finding the broken parts of a relationship. Little, everyday problems are seldom unique; in a world of billions of people and fourteen thousand years of recorded history, someone somewhere has had whatever problem you're having before. Experience is the best teacher, the saying goes..but sometimes the tuition is very high. Learning from other people's mistakes is less costly than learning from your own. Having a support network of friends who are close to you makes solving problems of all sorts far easier.
But what happens when a person digs himself a nice little cave at the back of the closet?
In extreme cases, he does exactly what an abuser would do to him, only he does it to himself. When a person refuses to share the reality of who he is with the people around him--even with friends and family--e does more than live a lie, and he does more than project a false façade to appease the prejudices of others. He cuts himself off from his support mechanisms; he isolates himself from the very people who might be there to say "Dude, that's fucked up!" if things start to go wrong. He creates barriers between himself and those people who might be able to help him solve problems or spot weaknesses in his relationship. He removes his own ability to bounce ideas off of others. He creates a breeding ground where unhealthy habits can fester and grow, unchecked by the light of day.
And that really sucks.
In discussions about the values of openness, i often see people arguing the perils and potential consequences of coming out. What I rarely see, though, is acknowledgment of the fact that remaining closeted has a price, as well.
And the more I think about it, the more I think the price of remaining closeted can sometimes be greater than what might at first be obvious.
The culture of secrecy can lead to a mindset of avoidance, of not talking about uncomfortable things even within the relationship. If one builds a reflexive habit of concealing the truth, it's hard to put down that habit even when talking to someone on the inside. At worst, in the most extreme cases, it can lead to precisely the type of dark, inward-burrowing isolation that the abuser seeks to impose on a victim, only self-inflicted and therefore even more internalized.
Your life is your own. It belongs to you and to nobody else. Live it as you will--but be aware of all the potential costs of your decisions.