I don't care if we legalize polygamy as long as they don't get more financial/tax benefits (or fewer) than couples. To me, equality in marriage means....equality.
I would say that it should be as scalable as having multiple children. i.e. each person still gets a standard deduction, or personal exemption or whatever.
That is not how bisexuality works. Instead it works similar to someone who is attracted to people of more than one race. Just because a person would be willing to marry someone of any race or either sex doesn't mean those people want to marry multiple people at the same time. Those who want to be in a group marriage are generally polyamorous.
To add to that, even being in a polygamous marriage, with both multiple husbands and multiple wives, it doesn't automatically make any given spouse bi-sexual. While many people cannot concept it, it is possible for one to romantically love another without being sexually attracted to them.
1. In the past such societies were almost exclusively polygamous (1 man, multiple women) and structured in such a way as to be abusive to women. Women were viewed almost as property and were expected to be subservient to the man.
This concept of women as property was pretty universal among both monogamous and polygamous socities. Additionally, you are viewing only polygyny as opposed to polygamy, the latter holding no concept to limits on one of the spouse's gender.
2. It was not uncommon for older men to exercise political (or religious) "power" over community such that very young women were forced into marriages with these older men (often much older) and left with no means of escape from the community. (i.e. statutory rape with no means of escape.)
Again, this is not isolated or even concentrated among polygamous societies. Arranged marriages were common with young women, even girls by today's standards, going to much older men. They had no means of escape from their monogamous marriage rape either.
3. High concentrations of polygamous marriages tends to skew the natural ratios of the available male/females in a given population. If you have one man marrying multiple women, those women are effectively removed from the - ah - market so to speak. Now you have an increased number of males while at the same time having a shortage of available females. Leading to problems with how to deal with the males who were often excluded from the community.
Higher concentrations of polygyny lead to these problems. Where actual polygamy is allowed, i.e. allowing for both multiple husbands as well as multiple wives, then the issue is much less. Additionally, homosexuality, bisexuality and even asexuality are factors in any imbalance there would be of available mates, assuming only monogamy.
In each bigamous marriage, there would be at a minimum three legally intertwined status:
A married to B,
A married to C, and
B married to C.
So there is a secular reason to be leery of bigamy as a government recognized entity that has nothing to do with religion or morality.
Do keep in mind that bigamy=/=polygamy automatically. If the law holds a single marriage status between all spouses then it is only polygamy without bigamy. Bigamy is holding multiple marriage status, i.e. the license. Now if the law handles polygamy by allowing multiple license, then yes both occur together.