I dont really like the term `relationship`, it smacks of the sort of semi-clinical PC talk that drains all the colour out of life, the same lexicon that brings us `partner`. Not that you mean it in any such sense sir, just a complaint I wanted to air!
This is a very broad query requiring a lot of conjecture, so I will avoid an attempt at a direct answer. The position of tradition is generally monogamous marriage, a human social institution which has both personal and societal elements. A childless marriage may be personally fulfilling to each participant, lots of things can keep such a union together, but it is a failure. It is less likely that it will last. A young and fertile wife is not an absolute necessity for success, it just improves your chances. Men are most strongly attracted towards women just before the prime of their reproductive life, that can give a man the best chance of plentiful, strong offspring in a long term union.
The greatest things that a woman can give a man (excluding metaphysical love) are children and beauty. With the latter fading with time, the lack of the former would become a sharp discomfort. Current western culture fetishizes romantic love and forgets about procreation, so I dont know whether this feeling is something considered evil, abnormal or some sort of betrayal. Is it?