Please don't write comments to the effect "actually, polyamory is a thing," because polyamory is much, much rarer than monogamy. Yes, technically, if you looked hard enough, you could find at least one person who is interested in polyamory. "Technically at least one" is an extremely low bar and is irrelevant. It is way harder to find someone to be polyamorous with than to be monogamous with.

tl;dr: Due to sexual monogamy being "standard" when it comes to dating, looks are way more important than personality. If open relationships were more widely accepted then maybe looks wouldn't be the most important thing when it comes to dating, but our society is not like that, and - practically - that will never change in your lifetime. If you think people should only care about personality, or you think that people who consider sex important are shallow, then you are living in a fantasy land.

You hear a lot of people saying that looks aren't the most important criteria used to choose a romantic partner. You hear a lot of people saying that personality is the most important factor. A lot of people look down on people who think sex is important, as if wanting to be with a physically attractive person is some personal moral failing.

Physical attraction is the most important thing for finding a relationship because the de facto "standard" relationship mode expects sexual monogamy. When you marry someone you are effectively signing up for, among other things, "this is the only person I'm ever going to have sex with for the rest of my life." That being the case, you're going to want to make sure that the person you are going to marry (whoever that is, or maybe you'll never marry) is the kind of person you'll want to have sex with for the rest of your life, to the exclusion of all others. You're signing up for never having sex with anyone else for the rest of your life; no one except for the person you're choosing to marry.

So, since this person is the ONLY ONE YOU'LL HAVE SEX WITH FOR THE REST OF YOUR LIFE, you're going to make damned sure they're someone you want to have sex with. That's why sex/looks are the most important thing for dating. Personality is far less important. When you're married you can (should) still have more friends than just your spouse. When you get married you don't have to break off all of your friendships and only spend time with your spouse. You don't have to give up all of your friends after your wedding. In contrast, after your wedding you DO have to break off all of your other sexual relationships / FWB / casual hook ups. The only one you get to keep after your wedding is the one with your spouse.

Exclusivity: for better or worse, that's your only option, and there's nothing (or next to nothing) you can do to change that fact. If you have zero interest in polyamory then that restriction isn't much of a problem. However, if you're anywhere in the range of "I might be open to the idea" to "I ONLY want polyamory" it will be practically impossible for you to get that. Not impossible strictly speaking, but for all practical purposes, it might as well be impossible for as much good it will do for you.

Over the last few decades there has been some small movement towards greater acceptance of polyamory, but the vast, VAST MAJORITY of the people you will ever interact with will only be interested in sexual monogamy. You don't really have much of a choice about it. Monogamy is the de facto standard. That's just a fact about the society we've all been born into, and it will not change in any significant degree over the course of your lifetime. Sexual monogamy is "the only game in town."

So, knowing that you can pretty safely assume that everyone you might want to date will expect you to be sexually monogamous with them, how does that affect the way you'll choose the person you want to marry? What you'll want to do is make physical sexual attraction a high priority (even if you wish you didn't have to). Maybe the highest priority, or maybe somewhere in the list of top 10 most important things you'll want in a spouse.

Imagine you're married for several years, things are going good, but then you tell your spouse "I've decided that I don't want to have sex with you any more and I'm going to start having sex with other people." How do you think your spouse would react? Most people will not be happy to hear that up to and including divorce.

For dating, personality is much less important than looks. Being married to someone doesn't mean you can't have any friends. You don't have to spend every waking moment with your spouse. It is healthy to have some away time, to socialize with other people. Anyone who insists that you stop spending time with your friends is not going to be a good candidate, it's a huge red flag. If the situation were more like "you're sentenced to life in prison, and this person is going to be your cell mate," then in that situation personality would be the most important factor.

There are many ways to express love, from sex to gifts to cooking for them. Most people would not expect their spouse to only eat food they cook, but they would expect their spouse to only get sex from them. You aren't "cheating" if you make yourself a sandwich, rather than your spouse making it for you.

Restaurants are a good analogy: let's say you had to pick one restaurant to be the only one you'll ever eat at for the rest of your life. In that scenario, which restaurant are you going to choose? It would be foolish if you didn't make sure it was a restaurant you like eating at. Just like how you'd want to make sure the food was good at the restaurant, so too would you want to make sure the sex you'll have with your spouse will be good.

No one's wife ever left them because he played tennis, or went bowling, or golfing, etc. outside of the marriage. In contrast, a lot of divorces happen (at least in part) because one person had sex outside of the marriage. It's an interesting asymmetry.

Finally, it's bizarre how sometimes one person in the marriage will lose interest in sex but at the same time insist that their spouse not have sex with others, to still consider it "cheating." Like, you're not even interested, so why would you care how your partner spends their away time?