In a monogamous relationship:

• The woman can withhold sex whenever she doesn't feel in the mood, and there's nothing you can do.
  
• The woman is more likely to cause drama, get in arguments, call you names, escalate physical violence, challenge your your decisions, and otherwise put you on the defensive.
  
• The woman is less likely to make the same effort to keep you around that she did when you started going out, because she knows she has you.
  

And here's the thing: you cannot predict how she will act several years down the road. She may very well be a sweet and awesome woman 10 years later, or she could be a heinous bitch. You won't know until you get there and by then you'll already be too invested to end the relationship easily.

Additionally, there's the issue of marriage and settling down. Let's look at the statistics, shall we?

For every 100 guys getting married in a given year, there are 50 other people getting divorced. That's a 50% failure rate over the lifetime of the marriage.
What's more, every single guy who got married did so with the belief that he could beat the odds.
No matter how smart you are, no matter how well you communicate with her, no matter what a good girl she is, no matter how alpha you are, you will still never have better than a 50% chance to make the marriage last.

I'm sure you're reading this and thinking "well, that doesn't apply to me."

YES IT DOES.

Every single guy who gets married does so because he believes he knows how to beat the odds.

And yet, 50% of them are inevitably wrong.

This, to me, says that it isn't a matter of skill, but of chance. Flip a coin or take a guess. Good luck!

Part of the reason for this is that you cannot control your partner's actions and decisions. Even if you're completely dedicated to making the relationship work, that's still only 50% of the battle. You cannot control her behavior. If she loses her interest and wants a divorce, there's nothing you can do!

Oh, and here's another tidbit for you: women initiate divorce between 66% and 90% of the time. So the long-term marriage survival is even further out of your hands than the 50% base failure rate would imply.

When a relationship ends, it hurts like a motherfucker. When a marriage ends it costs time and money in addition to the emotional pain.
If you have any children, you have very little chance of gaining custody of your children.

If you sat me down in front of a Las Vegas game and said "Here's a game that you have a 50% chance of losing. If you win you get to feel good. If you lose, you stand to lose all of your money, a large chunk of the money you would make in the future, and it will hurt like a motherfucker for up to several years," I would look you square in the eye and say, "Are you crazy? Of course I'm not going to play!"

And yet, so many people make this same decision every day, blindly trusting that they can beat the odds.

Don't be that guy.