Whatever else is going on in his head, you can find out only by A) asking or B) waiting to see what he does next. Personally, I belong to the waiting-to-see-what-he-does-next school of thought. After all, he might not have had anything else going on in his head before you had the coffee, but then something else pops in during the coffee. All the better reason to just relax and be yourself (your public self) as you drink your coffee.
The only way you can know that a man wants to marry you is that he tells you or one of his friends tells you. And if he is the kind of guy who starts thinking marriage from the first coffee, then it probably will be one of his friends who tells you. And he would not really be a man, anyway, but a superannuated boy. I suspect B.A. holds our generation's land speed record for courtship, but even he wasn't thinking marriage the first time we sat across a table from each other. It crossed my mind a few days before it crossed his mind, and I told myself not to be an idiot.
I mention this because I remember you telling me that there are guys who DO act as though an invitation to coffee were a marriage proposal, and that by saying yes to coffee with them, you have yes to eventual marriage to them.
Well, that's just silly of them. The most they should assume is that you won't actually faint or scream if they ask you out a second time. Meanwhile, every "yes" bolsters the confidence of poor old unfashionable NCBs, so even if later you find yourself facing blank incomprehension or even tears when you say "no" to date 2 or date 3, don't feel guilty. It is perfectly reasonable behaviour to have a cup of coffee with a man without having previously decided to marry him. Occasionally even married I go out and have a cup of coffee with a man, only in my case I have to tell someone else, i.e. my husband, first.
Dear me, the first time this happened. There it was: an invitation to coffee on my computer screen from a stripling of 23. I could not for the life of me determine his motive. I rose from my desk, squawking excitedly like Mrs Bennet in Pride and Prejudice.
"I don't know," said B.A. "Why don't you ask?"
"I can't ASK," I said, falling into a half-faint on the sofa.... Actually, I am making up this conversation, for I cannot remember what we said. At any rate, I arrived for the coffee in a tizz about a court case, and my generous host gave me a lecture on how corporal punishment should be reintroduced into the Scottish legal system, and how I was foolish to expect the lower orders to subscribe to middle-class morality and heretical to suggest they embrace Scottish Enlightenment principles when instead they should all convert to Catholicism. I argued weakly against all of the above, and went home for supper.
"I feel no illicit thrill from this coffee," I informed B.A.
"Well, you can't feel an illicit thrill if you tell your husband beforehand," said B.A., but nevertheless I always tell him before I have a coffee with someone. This is called prudence. Prudence trumps thrills. And I never did find out why my generous host asked me out for coffee because I never asked. Perhaps he merely thought he would enjoy having a coffee with me, so I hope he did. It was certainly an interesting conversation.
Really, when your imagination, hopes and fears run away with you, the best thing to do is to snap yourself to the reality of Right Now. How can you enjoy Right Now if you are thinking only of Maybe Then? In fact, perhaps you could put a hair elastic around your wrist and snap it every time you catch yourself dreaming about Maybe Then. I know it is difficult not to daydream when you are in your twenties, especially during class lectures on Euripides, but you really mustn't, or you will miss out on the fully lived experience of your twenties. (One of the wonderful things about being in your twenties is that people, especially older people, like you just for being young and attentive. All you have to do is go to Mass, and you will fill the hearts of aged Catholics with joy: "Look! A YOUNG PERSON! Mass is still found relevant by the YOUNG!" It's a sort of holdover from the 1960s--enjoy it while you can.)
Another tendency to watch out for is premature despair. I realize that youth seems to be bi-polar, overwhelmed with hilarity one moment, and in deepest depression the next, but this is no way to conduct your social lives. Until all the facts are in, there is no REASON for you to hope for a happy conclusion to your budding romance (if it IS a budding romance) or to despair that it will all end in misery and disappointment. You probably do not have enough DATA to make a good guess either way.
However, do examine unflinchingly any data you receive. If he says he's an atheist and thinks religion is the root of all evil, he's an atheist who thinks religion is the root of all evil. If he's had children with three different women, he's had children with three different women. If he's separated and hopes to get an annulment soon after he gets his divorce, he's still married. (It hurts me to say that, but it's true.) If he says he is a huge Audrey Hepburn fan, and all your friends tell you you look like Audrey, chances are, he thinks you're beautiful. If he drinks a bottle of wine a day, he drinks a bottle of wine a day.
It usually takes time to collect data, just as it usually takes time to forge a lasting friendship. Therefore, do not make up your mind, yay or nay, about a man the moment he asks you for coffee. Have the coffee to see if you enjoy having the coffee. This will give you a clue as to whether or not you will enjoy having the dinner.
Update: Today on "Ignatius Press Novels", Who is a Catholic Novelist?