Why multiple marriages won't work or why it is bad.
This section covers the counter-arguments against the suggestion, which I will try refute.

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If I'm married to multiple people, the value of my original one soulmate would decrease.
(what is also implied (but no one has said yet): I think that I would be valued less if my soulmate had more than one soulmate)
I do understand this argument, but the underlying issue here is how we each think of relationships and how we use them: how do you value people? Do you value them on the basis that they're your girlfriend/boyfriend or do you value them for who they are instead? If you weren't their soulmate, would they be worth less? Does it matter what formal, descriptive label we use for them?
My point is that a person's worth is not dependent on what you call them, be it friend, soulmate or whatever.

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Having more than one soulmate in the game is cheating.
Yes. If you would consider it cheating if you had more than one soulmate, it certainly is cheating. But there are other people who might feel a completely other way than you.
Could people coexist with multiple soulmates peacefully? Why? Why not?
Yes. I am pretty sure that some people would be a-okay with multiple soulmates.
A thing that I want to highlight make notice of, for this argument:
YOU DO NOT HAVE TO HAVE MULTIPLE SOULMATES

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It will create drama.
It will and it might be annoying to listen to, but there are multiple things going on when there is any kind of personal drama:
Some personal boundaries will be crossed. (which is quite an amazing thing - this is actually a mouse flash game)
In this case, someone might be a little sad that a person they held in high regard married someone else than them. Which is okay. People can do what they want. You might learn a thing about yourself and you might learn a thing about the people in your circles.

The second part of the argument (that it is annoying to listen to):
You are not obligated to read or listen to other peoples bullshit drama. You could ask them not to bother you with it AND if that doesn't work, there is a mighty fine ignore button that you should be acquainted with.

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Some people do not practice polyamory/polygamy and they do not tolerate it/they shun it.
Yes. While it's a shame that they do not understand it, this doesn't really .. take anything away from them. They don't lose anything if this is implemented. What other people think about you, or a group of people, is not important to you, personally. At least it shouldn't be.
Okay. Some people might actually quit Transformice because of multiple marriages. I want you to think about why they left and what they might think about people who have multiple soulmates.

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Thank you for reading, I think I've pathed out why I think the opposing arguments have flaws, in a concise manner.

But why multiple marriages?
Having gone through the arguments against, it's good to go through the arguments for:

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It's nice.
You can properly give a shout-out to the mice you love. Your relationships with your close mouse friends or lovers becomes part of your mouse identity. I would love to have two spouses and show other people that, I am happy and really appreciate them.

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It challenges people.
As I established earlier: some people think it's okay to have multiple soulmates and it is not cheating for them personally.
If naysayers could see that it works for some people, they might reflect on it themselves and come to a new conclusion (what is also called: learning something) about themselves. I'm not saying they might want to be polyamorous, but it can't be a bad thing to give it a thought or two.

It makes people reflect over something they haven't given much other thought in a new way.
That's a good thing. (at least in my opinion)

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It should honestly be a basic human (mouse?) right.
It doesn't harm the mice that are not in a polyamorous relationship. Mice should be free to love and marry whom they please.

What my main point of this post is: It will make this game better and we will all (players and developers alike) benefit from what is to be gained from such an adjustment to the game, be it monetary gain, or a better community or just a tiny, unconscious reflection by challenging people's thoughts.

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Thank you if you read all that. Maybe I'm taking this a tad too seriously, but apparently it's somewhat important to me. :~)
It was also good to flesh out my own thoughts out that I personally have about the "issue."
I am thankful for the opportunity to at least be discussed, even if it's not in a good tone or whatever. Dialogue is cool! It's good to hear different perspectives, even if I (or you) don't agree with them.