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Painful coitus may be due to thick hymen, vaginismus
, etc. which needs medical attention.
If it is not due to a medical condition, then most often it is due to dryness of vagina caused by insufficient sexual stimulation before penetration attempt and occasionally due to disproportionately large penis.
Spend quite some time in foreplay so that the vagina is adequately lubricated by the vaginal juices, use fingers to gradually stretch the vaginal opening, before attempting to penetrate.
If still there is problem, use K-Y Jelly
to lubricate the penis and vagina.