1. Legally there is no bar in two persons of opposite s*x living together without being married to each other,
2. However, it is also a fact that ours is a orthodox soceity and as you rightly envisaged, policre may arrive & try to harass you or book you in various sections of different acts,
3. I agree with you that, to be in the safer side from the police over action, you should execute a Maitry Karar/Friendship Agreement to show to police, should they attempt to disturb you,
4. It should be a simple notarisedc agreement, inter alia, stating that you both are staying together willingly without performing marriage and are free to leave the relationship anytime without any notice.