I think a baby can keep partners together longer but it doesn't have to do so much with the love between the two.
In most love affairs after the period of 'being in love' the two tend to become a little alienated with each other. If a baby appears the tenderness of the parents is focusing on the baby while the exhaustion of everyday work tends to make the partners emotionally colder to each other.
To summarize, i'd say that no, i don't think a baby is a reason to prolong the love between two partners; but in the case where love is replaced by routine and job-slavery a baby is the best reason to keep them two together for some time to come.
Love and understanding is the result of a spiritual bond and depends upon the two who decide to become complementary to each other.