No one has ever accused Darwin about making a rush to judgement about any topic. Just as he spent years poring over the minutest detail of barnacle anatomy before he published The Origin he gave the topic of marriage careful consideration before singing on. In fact, preserved in his notebooks we have a record of the deliberations he undertook. Sometime in 1838 Darwin turned to a new page in his notes and drew a line down the middle, he added the headings "Marry" and "Not Marry" to either side of the line an proceeded to list the pros and cons of either decision. You can see the notebook here but below (presented without comment) is a transcript :

Marry

• Children — (if it Please God)
• Constant companion, (& friend in old age) who will feel interested in one
• Object to be beloved & played with —better than a dog anyhow.
• Home, & someone to take care of house
• Charms of music & female chit-chat.
• These things good for one's health.
• Forced to visit & receive relations but terrible loss of time.

Not Marry

• No children, (no second life), no one to care for one in old age.
• What is the use of working 'in' without sympathy from near & dear friends—who are near & dear friends to the old, except relatives
• Freedom to go where one liked — choice of Society & little of it.
• Conversation of clever men at clubs
• Not forced to visit relatives, & to bend in every trifle.
• To have the expense & anxiety of children
• Perhaps quarelling
• Loss of time.
• Cannot read in the Evenings
• Fatness & idleness
• Anxiety & responsibility
• Less money for books &c
• If many children forced to gain one's bread. (But then it is very bad for ones health to work too much)
• Perhaps my wife wont like London; then the sentence is banishment & degradation into indolent, idle fool

It's hard to make this add up with his decision.