The biggest factor that determines how good sex is is how much you want it - both during anticipation and during the act itself. Sex tends to be best when there's a lot of desire and sexual tension - building up lots of extra desire through delaying or making its fulfillment uncertain. (And btw relationships tend to kill uncertainty around whether sex will happen or not. It becomes a foregone conclusion, which makes it more boring.)

Since desire is central to what makes sex good, I want to look at sex through the lens of the field of marketing because marketing is largely about what drives human want. In particular, there are four ways that have been identified as factors that make someone want something:

1. Show other people like it - This is people using the vetting process of other people as a heuristic for what is good, so we don't have to vet each and every thing ourselves. Example: "I have no clue what that restaurant is, but it's lined out the door and down the street. I wonder what all the fuss is about..."

This is a tricky one when applied to male desire for sex because most men will be massively turned off if they find out the girl they're talking to is an instagrammer with 15k male followers (due to men being attracted to purity - see #4 below), but it still absolutely applies. The distinction is in her actions to attract male attention vs his actions to attract certain carefully selected male attention to her (his actions don't subtract from her purity, obviously). Men will hate when women do things to gratuitously draw male attention, but at the same time we will totally show our friend that we trust not to steal our girl pictures of the hot girl we met last night and drip feed off his jealousy like a thirsty gerbil. The fact that our friends are jealous of the girl that we're with is both validating to us and makes us want the girl more.

Now to compare to post-relationship: Let's say a married guy makes a new male friend that enters into his circle of trust. Do you think he's pulling out his phone and showing this guys pictures, saying "Check her out. I married this woman. Sexy, right?" Not unless he has a very specific kink. No matter how much he trusts his new friend, the boost he'd get in validation and desire from another guy's approval isn't worth the hit to his sense of security he'd get now that he's committed, so men don't do this anymore. It's a tradeoff that's worth it for us, but it still reduces our desire because we're married to a woman that's essentially kept out of bounds as much as possible to the judgement of other men, even our closest friends.

2. Make it scarce - A sense of scarcity leads to a fear of missing out if you don't take advantage while you can, as opposed to thinking you can do it any time and so you never do. Example: "You bought that stupid boat after your brother let you borrow his for the weekend and you said you loved it, but now that you can go out on the lake any time you want you haven't gone even once!"

When applied to male sexual desire, this is THE thing IMO that drives about 75% of it (but very, very few men will admit this because doing so makes you unattractive to women automatically). Men want to have choices. We learned early on that sex is the only form of female relationship investment that actually means anything, so the choices we have are equal to the women that are willing to sleep with us. And no one wants to marry a woman by default because she's the only woman that would sleep with him. We want our choices to be meaningful. That's why men envy players - they have choices. That's why we try to become players, even if it means dumpster diving and sleeping with frogs that we end up being ashamed to tell our friends about (So 0 value in #1 and #4). Scarcity drives it all. It's ugly and you can't admit to it but by far the most powerful force in driving male desire in a dating environment.

In a relationship, there is no scarcity. She's there - always available. This kills a huge part of desire.

3. Make 'em work for it - Something you have to earn feels more meaningful than something you get for free, because what you prove about yourself in earning it becomes part of its value to you. Example: The string dangles just outside the cats reach, dangling enticingly. As she lazily watches it wiggle, she starts to wonder if she's fast enough to catch it. Then the string flops forward, landing lazily right on top of her paw and she loses interest completely.

Dating is THE proving ground for men. There's such a big overlap between the traits that women find attractive in men and the traits we think of as aspirational in general - eg, money, power, influence, etc - (I don't think this is a coincidence at all btw) that it's no wonder that so many men see the sex they're able to get as the most straightforward way to measure their own value. This is why men often brag to each other about the number of women we sleep with or how they look, but we seldom brag directly to each other about things like how much money we make or how high we are in the hierarchy of power we're in. We know other men think of the latter mostly as just a stepping stone for the former, so it's always better to just brag about the women you've banged. Also, women's sense of security comes from asking for investment from men. They love to make us work for it in the beginning - time, money, effort, attention, etc. All of this works together to make the eventual sex feel like something we earned, making us want it more.

In a relationship, you don't have to earn anything anymore. There's a reason people tend to start going to the gym when they become single and become couch potato blobs when they're in a relationship. The other person is, to a certain extent, stuck with you. That's the main feature of commitment. Even if you let yourself go or stop trying as hard to be your best self as when you're dating, the other person won't leave you. That's where the sense of security comes from. It's what lets you be your lazy, relaxed, comfortable self instead of your try-hard, top form, best self. But it drives down desire (not to mention she might if she does the same and becomes a couch potato blob too lol).

4. Demonstrate its value - This is about the person's awareness of how much better their life would be if they had the thing vs not. Example: "Yeah, I know a new phone is expensive but my old one broke and I rely on my phone for everything! I'm not going to not have a phone."

This is basically where all the individual variation lies (and why some men will disagree with the thread title). Different men get different things from sex. Everyone's needs are calibrated differently. Some men will come out preferring relationship sex over casual sex because they value a deepening emotional or companionship connection, better consistency, and/or someone who actually learns and remembers what they like. Others will prefer casual because of novelty, variety, and the thrill of the chase. The thing that pretty much every man will value is fulfillment of his spontaneous sexual desire from a partner that he finds attractive. Men tend to find the same attributes attractive (looks, age, purity, personality) but there's a lot of individual variation in how much importance we assign each of those. Someone who values looks or age more will find grim prospects for a relationship, whereas someone who values personality more might be more optimistic.

What's important here is that, when women think about a man's desire for them they tend to only think about #4. For women, sexual desire is not driven by scarcity because sex is not scarce for them. For women, sexual desire is not driven by working for it because they usually make us do all the work. (Women are massively affected by #1 though IMO) So women forget #2 & #3 at least even exist. The desire I've felt in every relationship I've been in has only taken a nosedive the deeper into commitment we've gotten regardless of who the woman was. And I think that's a common experience for men. In some cases, it's a conscious tradeoff.

TLDR: Men have major factors which don't affect women that drive our sexual desire (scarcity and investment) that will always be stronger at the beginning of a relationship and be reduced the deeper into commitment you go.