In Macbeth, how do Banquo and Fleance help with Macbeth's manhood?
This is a very interesting wording you use in your question. To what extent is Macbeth's decision to have Banquo and Fleance killed about his "manhood"? Well, I suppose we could argue that in terms of his facing up to situations and problems he is facing, his decision to have them killed represents a move away from having his wife do his plotting for him towards him dealing with his own problems and doing his own plotting. Let us remember what Macbeth says about Banquo in Act III scene 1:
To be thus is nothing;
But to be safely thus.--Our fears in Banquo
Stick deep; and in his royalty of nature
Reigns that which would be fear'd: 'tis much he dares;
And, to that dauntless temper of his mind,
He hath a wisdom that doth guide his valour
To act in safety. There is none but he
Whose being I do fear: and, under him,
My Genius is rebuked; as, it is said,
Mark Antony's was by Caesar.
Macbeth therefore identifies that Banquo is a problem because of the "royalty of nature" that he has and his "wisdom." It is clear that he fears Banquo will realise that Macbeth was behind Duncan's murder and that his honesty and values will lead him to reveal this information. He is therefore a threat to Macbeth's ambition. In this sense, Macbeth is forced to show his manhood with how he deals with the problem rather than leaving his wife to do his thinking and plotting for him, as was the case with Duncan. Of course, we would also say that this is where Macbeth shows more of his villainous instincts as well, however.