There is a fundamental right given by the Divine to make our free choices and suffer the good or bad consequences. That is the whole basis of the law of karma. It is a Divine arrangement and nobody is given the right to interfere with this process unless one is a child who is not equipped to take decisions or a student who has given responsibility of guidance to his teacher and thirdly, the choice, the actions are likely to harm the individual or the society at large. This is not the case here. Parents can discuss the possible fallout of late marraige or staying single but there is a difference between this and wanting the child to get married for their emotional happiness.
Besides how does one know what the future holds and what is really good for the other person? Is the person pushing one’s decision onto another ready to take the possible consequences. To say that chosing not to marry is selfish is like saying not buying a car is selfish because others want to travel in the car you are being pushed to buy against your choice! Car is still a physical object. But to bring home someone’s daughter when the boy is not ready is both selfish and insensitive. Marraige is not a doll play but a serious life long committment and one should enter into it only if one is ready for it fully.