People figured out thousands of years ago that marriage is good.

Thousands of years ago marriage consisted of one man and multiple women. Mostly the women were bought from their fathers, they had no say in who they were going to marry. Also it was common to take the virgins of the people you conquered in wars as you wives or concubines or sex slaves. Marriage was good for the man, not so much for the women.

In general though, most people are honest and have good intentions and are not trying to take advantage or abuse other people, so most marriages should work fine.

But they don't. As I said people just fall out of love.

At least in Christianity they offer the right to divorce for reasons such as adultery, but divorcing someone doesn’t mean you can hop into bed and shag the next person.

I don't care what your god has to say though. I don't believe in your god and I am against forcing people to believe in your god and doing things your god commands you. You obey your god and as long as that doesn't involve other people I don't care.

People need to get over biology and have real purpose.

They get to determine what their purpose in life is, not you.

One man, one woman applies to Christianity but not to Islam.

Have you ever read the bible? It sounds like you never did. For example

2 Samuel 12:11-12

Out of your own household I am going to bring calamity on you. Before your very eyes I will take your wives and give them to one who is close to you, and he will sleep with your wives in broad daylight. 12 You did it in secret, but I will do this thing in broad daylight before all Israel.’”

Here because David had sex with Batsheba God decides to punish him by having his wives raped. Later on his own son indeed rapes all of his wives on the roof in the view of the entire town.

Infidelity is only rampant because people have Reddit levels of belief in decency.

Maybe they just don't believe in your god. Have you ever considered that?

If people are shunned and shamed then they won’t engage in that behaviour.

Why should they be shamed or shunned for mutually consensual sex?

That’s why they used to kill people for infidelity, it weeds out the psychos who are naturally impulsive and inclined towards evil.

Depends. If you read the bible god did indeed order the stoning of women who commit infidelity but the men were never commanded to be killed. At worst they paid some money. If they raped a woman their punishment was to marry that woman so that they could continue to rape her legally for the rest of their lives.