hi Swetha,thanks for query.Normally even after couple of delivery the vagina opening returns to normal size and does not lead to any problem during sex.Still if you feel you are not comfortable please visit a gynecologist and confirm it and then you may opt for plastic surgery
which can take care of this and make this opening smaller.
In mean time while having sex keep your legs crossed on his back in man top position this will make him feel the grip.Do regular deep breathing exercise
and exercise of your perineum
muscles which will definitely help.This like holding your urine or stool,you can feel the tightness in bottom.wishing you good health.