Their marriage may or may not last. I sincerely hope their marriage and love lasts for the life.
My main issue is that apart from social problems, there are many legal problems such marriages would lead to like
Will one of them convert to the other religion?
If he coverts to Hinduism, he would be accused of apostasy by Muslim leaders while on the other hand if she coverts to Muslim, he would be accused of Love Jihad by Hindus.
Assuming both of them keep their religions, Being a Muslim what if he marries another girl and she being a Hindu takes him to court?
If she continues to be a Hindu and let assume she is eligible for reservation. What would be the status of the their children? What religion and what reservation can they claim.
Neither the religious/social systems nor the legal systems in India are compatible and mature enough to handle such situation.
Hence, I do not encourage such marriages.