Hello,
No this can not be challenged by a third party, but yes I would not advise you to do so as doing the same is an offence.
At a later stage if anyone of you gets married again and there is dispute again in matrimonial life then the other party can use this as a strong point.
Also, you may just ask them to sign agreement and file the case after 1 year of marriage and till then live separately.
Regards