Assuming that he was even considering a relationship...
The thrill of "the chase" and the fantasy of it is really good, like seeing the dessert tray before dinner!
Anticipation of intimacy is the great buildup of tension and sex is the release of it. Trouble is, once you've had it, what is exposed is all the other relationship stuff and possibly that the sex was just ok. Realize you are competing against his dreams, fantasies, other girls, etc.. Relationship questions surface such as distance, her situation, difficult x husband and kids, stresses, financial, can this really work, etc.. I'm not talking about physical imperfection so much, but everything gets rolled up into... what do I do next? Do I keep this relationship or wait for another?
The problem is... his needs are now satisfied... the sex drive, a major thing that propelled him towards the female... is done (for the moment), so he's dealing with all the other stuff. And he may look at the whole thing in sum total and determine... I'm not sure, I just don't know, maybe, kinda?... Guys think about things and then stop thinking about it and move onto something else, especially if they can't reach a conclusion easily. Girls can't stop thinking about things like this and it spins round and round in their minds.
If it isn't a definite yes, then the answer is often a maybe or a no, which means it's gonna fall apart because the pursuit, effort, texting, etc.. drops off? This is where the girl, if she is interested, and they usually are because girls often are more bonded during sex, starts to freak and ask questions...why isn't he texting me (communicating)? Its been 6 hours and no word?
By releasing the tension on something that is the major bait for the guy, you are risking losing his interest if a relationship isn't established first because the release exposes other relevant questions which may kill the relationship... or dull it.
Realize there are other more important things valuable about you that draw him in and these being in place will help to keep him engaged in the relationship. Some examples are, he likes you as a person, you're fun, likes to be with you, you are helpful, just wonderful to be with, he sees woman in you, great with kids, can see a future with you, low stress, common interests, can learn from you or contribute to your life, you play together, emotionally healthy..he can handle you, you respect him, etc...
Before the sex, he can't really see the other stuff so well because the desire for you is so high...he's blinded. So you are supposed to take it slow, get to know each other, build romance, and discover those compatibility/incompatibility things and decide if this is gonna work... before mating bonding! If so, then it opens up the door for intimacy... because there is already a good chance you are moving towards LTR and sex is a compatibility issue (values system may determine when this is appropriate).
There's risks to mating before marriage as well as for waiting until marriage and you accept them as an adult in your relationship and figure out what is right for you.