In the second quatrain the speaker states his belief that the Dark Lady is trying to persuade the Young Man to have a relationship with her. This, of course, drives the speaker mad ("to win me soon to hell"). The situation is a living hell for him. The speaker describes the Dark Lady as a powerful devil who is trying to tempt or seduce the most beautiful and innocent of angels (the Young Man) to an act of evil (to sleep with her). The speaker is not so much jealous as he is concerned for the safety of the Young Man. The speaker fears the effect that the Dark Lady will have on the Young Man. This is not a matter of sexuality. Earlier (beginning with "Sonnet 1") the speaker had urged the Young Man to have relationships with women so that he could have children. But the speaker does not want the Young man to have a relationship with the Dark Lady, specifically, because the speaker feels that she will corrupt him. Somehow, she will make him evil like herself. He will lose his "purity" and be contaminated or stained by her "foul pride."
The speaker admits, in the third quatrain, that he is not sure whether the Dark Lady and the Young Man are actually having an affair. The speaker only does "suspect" that they might be having an affair because (1) he knows that they are friends to another and (2) he realizes that both of them are away from him at the same time. The speaker is only making a "guess." He does not know for certain. This brings up an interesting question. Does the speaker really have a good reason to suspect