Remember that Austen had her chance to marry, too, in very similar circumstances to Charlotte Lucas. She was 27, and considered pretty much on the shelf (she writes somewhere in a letter about going to assembly balls where she is no longer expected to dance: 'I am put on the sopha near the fire and can drink as much wine as I like'), had only a tiny income, and with her mother and sister Cassandra, was entirely dependent on her brothers, living on an endless round of long visits to family and extended family, with no settled home.
Her suitor was her friends' 21 year old brother, Harry Bigg-Wither, with whom she and Cassandra were staying. Jane said yes at first presumably purely on economic grounds (she would have had a home, she could have given a home to Cassandra and her mother) and the knowledge that he was a decent young man, though stolid and a bit of a bore clearly had the horrors overnight, changed her mind, declined the proposal, and then left first thing in the morning.
It wasn't nothing to decline an offer like that, from a decent and rich man, especially when she knew it was almost certainly her only chance to marry, and when her own circumstances were so poor and unsettled.
But she also saw in her sisters-in-law and extended family the cost of childbearing to women. Her six brothers on average lived until the age of 75, her SILs to an average of 45, and childbirth was associated with three out of the six deaths. She says somewhere in one of her later letters, about a niece repeatedly pregnant -- 'Poor animal, she will be worn out before she is thirty.'
Deciding not to risk that was an intelligent decision.