It is certainly true that Jane Austen seldom presents her gentlemen talking amongst themselves with no lady present, and when she does so, their conversations do not last long. She is particularly wary of writing dialogue between men.
This passage from Emma presents a summary of a conversation between Mr. Knightley and his younger brother, Mr. John Knightley, but without any direct speech:
The brothers talked of their own concerns and pursuits, but principally of those of the elder, whose temper was by much the most communicative, and who was always the greater talker. As a magistrate, he had generally some point of law to consult John about, or, at least, some curious anecdote to give; and as a farmer, as keeping in hand the home-farm at Donwell, he had to tell what every field was to bear next year, and to give all such local information as could not fail of being interesting to a brother whose home it had equally been the longest part of his life, and whose attachments were strong. The plan of a drain, the change of a fence, the felling of a tree, and the destination of every acre for wheat, turnips, or spring corn, was entered into with as much equality of interest by John, as his cooler manners rendered possible; and if his willing brother ever left him any thing to inquire about, his inquiries even approached a tone of eagerness.
There is, however, a brief passage in Mansfield Park which is often cited as a rare instance of conversation between two men in a Jane Austen novel that does contain direct speech, though not dialogue, since only one of the parties speaks. A conversation between Edmund Bertram and his father, Sir Thomas, is described as follows:
Edmund's first object the next morning was to see his father alone, and give him a fair statement of the whole acting scheme; defending his own share in it as far only as he could then, in a soberer moment, feel his motives to deserve; and acknowledging, with perfect ingenuousness, that his concession had been attended with such partial good as to make his judgment in it very doubtful. He was anxious, while vindicating himself, to say nothing unkind of the others; but there was only one amongst them whose conduct he could mention without some necessity of defence or palliation. "We have all been more or less to blame,'' said he, "every one of us, excepting Fanny. Fanny is the only one who has judged rightly throughout; who has been consistent. Her feelings have been steadily against it from first to last. She never ceased to think of what was due to you. You will find Fanny everything you could wish.''
The word alone in the first sentence clearly suggests that not only were there no female participants in this conversation, but no woman could have overheard it, either. It is often said that the lack of man-to-man conversations in Austen's novels is a mark of her realism. After all, if she was present during a conversation, it could not be exclusively between men. However, this idea seems to discount the plentiful opportunities Austen would have had for overhearing conversations between her father, her brothers, and their friends in a busy rectory.