The person who marries for money usually earns every penny of it

The person who marries for money usually earns every penny of it.

...anonymous quote.

What does this phrase mean?

It seems to suggest that if you marry for money, you will earn all of the money you married for, which would be a lot. That doesn't make sense.

The person who marries for money usually earns every penny of it.

Marriage is hard. This has been true throughout history; Socrates said

By all means, marry. If you get a good wife, you'll become happy; if you get a bad one, you'll become a philosopher.

The person who marries for love has their work cut out for them, because marriage, even with deep love, is still work. But deep friendship and love for and from each other is their reward.

The person who marries for money will not have love, respect or even necessarily friendship to see them through; all they have is money. Money is not enough. The novelty of material possessions wears off. The work of staying married to a person one doesn't love is much harder than where there is love. Their work is so hard, they will earn every penny of it.

The definition of "earn" on Google is "obtain (money) in return for labor or services." In other words, to earn something is to work for it.

I might say that I was given a gift by a friend, but I would not typically say that I "earned that gift". When I receive money for my work, then I say that I earned the money.

By this definition, the saying implies that the person who marries for money will get it in return for "labor or services", that he/she will work for it. This implies that being in a marriage solely for money is like working for money. Another way of looking at it would be to say that being married for the sake of money is like working a job, and no one wants their family life to be a "job".