The flaw in the logic of comparing polygamy and gay marriage is that an individual does not choose to be gay, an individual is born gay. People are not born polygamists.
Of course people are born gay. But people are not “born polygamists?” There are lot of poly people who will tell you or that they are naturally (born) poly. There are other people who are denial but haven’t stuck to celibacy or monogamy, being with many people. Some people are heterosexual, some gay. Some are poly, some monogamous.
Gender and number certainly are two different factors in the freedom to marry. Recognized polygamy has a long and rich history around the world. Marriage between people of the same gender hasn’t, because of so much bigotry against LGBT people. So yes, there is a difference. The freedom to marry one person of the same sex is not the same as being able to marry two or more people of whatever genders. But both of these freedoms to marry are essential to marriage equality. Let’s show some solidarity. The heterosexual person doesn’t truly have the freedom to marry if that person wants to marry two or more. The gay person or the bisexual person doesn’t truly have the freedom to marry if that person wants to marry two or more. This is why we need full marriage equality.
There is no good reason to deny an adult the freedom to marry any and all consenting adults.