If she had been poor, he would have urged her to marry him, although it might have exposed her to some risk of hardship.

But she was rich, and the best he could hope for was a post at a mine like the Clermont.
He had no ground for imagining that Agatha would be willing to marry him; but if she were, it would look as if he meant to share her riches when he offered his help.

In fact, it would look as if he meant to take advantage of her ignorance about mining matters and her trust.

It would not disturb him if outsiders thought this, but she might come to think so.
Besides, he was not going to be supported by his wife's money.

In view of their characters, the situation would be humiliating for both.