Well, it is hard to say why it should not be accepted legally.If some one is arguing against it on moral grounds, as Deepa mentioned, many things have been happening in India which are against the Culture/Morals/Dharma.Yeah, If they are legalised they will get all the rights as married couple have so that they will have a security in a relationship, as this is the case in few western countries.In reality, it complicates the life of kids and eventually the future generations.
P.S I have found an interesting view on morality from Mahabharath (online) which is the case in
present day world .Just want to share
(Bhisma’s reply was evasive....(when Droupadi questioned him about her insult in the court)
“..the course of morality is subtle. Even the illustrious wise in this world fail to understand it always. What in this world a strong man calls morality is regarded as such by others, however otherwise it may really be; but what a weak man calls morality is scarcely regarded as such even if it be the highest morality. From the importance of issue involved, from its intricacy and subtitlity, I am unable to answer with certitude the question thou hast asked…”
( Source:https://aavesh.wordpress.com/tag/uncategorized-tags-bhishma )