It is already legal to have a sexual relationship with multiple partners at the same time. Bigamy should be legalised. That it isn't, is a result of entrenched bigotry and religious prudery.
If you're using God as a proper noun, capitalise it. Not capitalising it is potentially confusing. Eg. There are numerous gods. Zeus is a god and God is a god.
It is likely too late to avert climate apocalypse. Don't have kids and you've done far more than most to prevent climate change. (This doesn't mean we shouldn't try to prevent it.)
A lot of society's ills are attributable to arseholes not getting punched in the face enough. This includes the fact that we can't make it legal to punch arseholes in the face, because arseholes would abuse that right to punch people who don't deserve it in the face.
America has become so fat, that most Americans have a skewed perception of what it means to be obese.
Most sports are stupid. Michael Phelps can swim 6 mph. Fish can swim 10x faster than that. Fuck that. I want to watch a tiger chase Trump through a maze.
The already high male suicide rate is severely underestimated. A lot of male risk taking behaviour, including car/bike related accidents, is suicidal in nature.
In practice identity based politics is often used to divide and conquer and distract people from their true interests. Class consciousness is more important. You have more in common with your colleagues, than you do with Beyonce or Joe Rogan, just because they happen to have the same skin colour and/or genitals as you.
The problem with bigamy is that most historic implementations of it involved a series of 1v1 marriages to one person who could marry others and the rest that couldn't. To prevent chains of marriage, only one gender was allowed to marry, usually male.
If you remove gender from marriage, you can get much messier marriages and there is no legal understanding of what happens in those cases.
None of these are unpopular takes other than the first one. It's no one's business who you fuck. We have no fault divorce for those who want out for any reason. The state doesn't care about why you divorce, and it seems like many successful people have multiple families so it also seems like it wouldn't be too difficult to trip past that either.
Edit: the climate change one is also unpopular since all that will happen is the loss of diversity and people who live near the shore will have to move inland. There will be more extreme weather events but it's not like we're going to turn into Venus. Things are just going to get more expensive and classist and racist.