They had accepted their acquaintance as too short for an engagement, but they had treated it as long enough for almost anything else, and marriage was somehow before them like a temple without an avenue. (22.214.171.124)
Merton and Kate want to get married, but they both agree that they haven't been seeing one another long enough to tie the knot. For this reason, they view the institution of marriage as a building they want to go into but can't because there's no road leading to it. They know where they want to end up, but not how to get there.
Only now he was having to think if [poverty] were prohibitive in respect to marriage; only now, for the first time, had he to weigh his case in scales. (126.96.36.199)
Merton has never thought of his financial position as something that would keep him from getting married. But now, with Aunt Maud in the picture, he realizes that this is definitely the case. In all honesty, it's kind of sweet and naïve for him to think that love is all that matters in a world filled with people like Aunt Maud.
"Will you settle it by our being married tomorrow—as we can, with perfect ease, civilly?" (188.8.131.52)
When he gets tired of waiting, Merton asks Kate to marry him outright. He knows that Aunt Maud has another guy in mind for Kate already, and Merton is on the verge of spending several months in New York. Before he leaves, though, he wants to marry Kate. But Kate tells him that they should hold off.
Maud Manningham had made, she believed, a great marriage, while she herself had made a small; on top of which, moreover, distance, difference, diminished community and impossible reunion had done the rest of the work. (184.108.40.206)
When thinking about visiting her old school friend Maud, Susan Stringham reminds herself that Maud married rich while she (Susan) married an everyday Joe. This difference in social position—as well as the distance between the United States and England—has made it difficult for them to stay in touch.
"It's open to her to make, you see, the very greatest marriage. I assure you we're not vulgar about her. Her possibilities are quite plain." (220.127.116.11)
Maud is frank about telling Merton that he has a shot with Milly. She admits that Milly can marry absolutely anyone; she doesn't need her to marry someone rich and powerful the way Kate does. In this way, Maud actually suggests to Merton that he should try his luck with Milly.
One was that even should he desire her without a penny she wouldn't marry him for the world. (18.104.22.168)
Milly has no interest in marrying Lord Mark. For starters, she thinks that the dude is just after her for her money. But even if he weren't, she'd have no interest because she thinks he has the sex appeal of a potato.
He came to make Milly his offer of marriage—he came for nothing but that. (22.214.171.124)
Lord Mark travels all the way to Venice to make Milly an offer of marriage. As we later find out, though, he does this because he knows Milly is terminally ill and he hopes to inherit all of her wealth. Sure, marriage can be a wonderful thing. But when there's money on the line, things can turn super ugly super fast.
"Does she [Aunt Maud] mean he has been encouraged to propose to her niece?" (126.96.36.199)
It sounds like while Merton is in Venice, Aunt Maud has been encouraging Lord Mark to propose to Kate Croy back in London. Merton is really annoyed by this, because he's not altogether sure whether Kate will turn down the offer. This lack of trust really doesn't bode well for Kate and Merton's future marriage (if there is one).
"And to lie myself, you mean, to do it? We are, my dear child […] I suppose, still engaged." (188.8.131.52).
It's quite simple, really. Merton will not lie to Milly by denying his engagement to Kate. Throughout the book, he has been willing to avoid telling the truth. But that doesn't mean he's ready to totally lie to someone's face.
"[We] shall only wonder at our past fear. It will seem an ugly madness. It will seem a bad dream." (184.108.40.206)
Merton promises Kate that if they can stick together and announce their engagement, everything will be fine. They've spent the entire novel in fear, but he's confident that if they take the plunge and call Aunt Maud's bluff, she'll totally cave and give them her blessing.