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When we say that someone is monogamous, we usually mean that he/she is sexually exclusive with one partner. But does that mean only intercourse or all sexual acts? Does that include emotional intimacy? How about cuddling or other nonsexual types of intimacy? Since we relate to people in so many ways, how we draw the boundary between monogamy and non-monogamy varies from relationship to relationship. It turns out that monogamy is not a binary, any more than polyamory can be described as simply the opposite of monogamy. Both monogamy and polyamory are on a continuum with multiple dimensions, which I’ll describe here as social, emotional, physical, sexual, and familial.
Social:
Humans are social creatures, and even though most of us want to pair up with a special someone, we often maintain social bonds with others. Do you go out to dinner, see a movie, go hiking or shopping with friends by yourself, or do you prefer to do those things with your significant other? People who are socially monogamous feel that forming a social bond with a person of the opposite sex (or same sex if homosexual) is a slippery slope to infidelity. Therefore they may prioritize socializing with other couples, keeping very transparent and casual all relationships with the opposite sex, and socializing as a unit as much as possible.
Emotional:
Sometimes friendships turn into deep emotional bonds and couples find themselves having to negotiate to what extent they feel emotional intimacy with others is acceptable. For example, would you be ok with your partner having a close friendship with his ex-lover? Would you be ok with your partner forming a close friendship with a person of the opposite sex? Would you be ok with your partner saying, “I love you,” to someone of the opposite sex? Some emotionally intimate couples are purely platonic while others develop romantic feelings. How would you feel about your partner being romantically involved with someone without sex? Do you need emotional exclusivity with your partner?
Physical:
Not all physical ways of relating are sexual, and they may or may not be within the bounds of a monogamous relationship. Some individuals are very affectionate and can kiss, hug, and cuddle with their friends and it’s not at all sexual. Some cultures are more physically expressive than others. Some monogamous couples are fine with their partners hugging and even flirting with others, but draw the line at kissing. Others may engage in massage or sensual touching but agree not to have sex with others.
Sexual:
We tend to think of sex as the last stop on the monogamy train. Some people need sexual 
exclusivity to feel safe with their partner, even when they are permissive in all other areas. For others, sex is not the ultimate symbol of love and devotion, but emotional intimacy is. One person may feel that “it’s fine for my partner to have sex with someone else, but I’m the only person who is allowed to cut his hair!” Some couples reserve specific sexual acts with each other or permit certain ones with others. For example, a couple may decide that BDSM with other partners is ok but they will only make love with each other. Some couples are ok with their partners having sex with others but don’t want them to sleep with other partners or go on vacation with them. Swinging is considered to be the type of non-monogamy that is sexually open but reserves emotional intimacy for the primary couple.
Familial:
While love may be infinite and potentially shared with an unlimited number of individuals, time, space, and money are limited and we may be able to share them with only one or two individuals. It is quite common that individuals who are polyamorous in all aspects may only share finances, parenting, or cohabitation with one partner. In those cases extra partners are like friends of the family or extended family. If other partners become integral members of the nuclear family and they become exclusive with each other, this type of arrangement is sometimes called polyfidelity. Even with people who consider themselves totally polyamorous, not every partner can be equal when it comes to the limited resources of time, money, and space.
As we can see, monogamy is not as straightforward as we may think it is. A couple may be emotionally monogamous but not physically or sexually so. Or they may be sexually exclusive but physically and emotionally open to others. Polyamory also has social, emotional, physical sexual, and familial dimensions. It is important to ask specific questions and understand each other’s level of openness instead of assuming we know what someone else needs. Understanding our own and other’s boundaries can also help us stretch them and grow in directions that will benefit us and our relationships.