so if some lady got pregnant and then the husband died, the lady should terminate the pregnancy?
One is an unexpected event, other (sleeping with someone for the sake of getting a child) is deliberate act.
Terminating pregnancy is also by large not a positive act or may not be possible.
It still carries risk to her health.
Being married and now widowed, the husband not only had given name to child but most likely some financial support.
Sleeping with someone for this sake has huge risk of the man running away.
If she is rich, stable, single mother, she lost further chance of getting married or may have to lower her expectations further to get married.
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Even if society accepts it, an individual person may not.**
And we have not heard of any marriage yet-temporary or permanent.