Let me share you my personal story. I used to be in a relationship which lasted 10 years. That was my first relationship.
First Month
For the first month it was good, getting to know each other well and doesn't expect anything in return. I was practically in a good mood I guess.
Second Month
When it comes down to our second month, I told my partner that if there will be a chance that he will like someone else in the future, he should be truthful and tell me about it and I will gladly set him free. For the least all I want is a relationship with respect regardless of any reason we depart. I really aim for that. It is a small world after all, do not want anything grudges or bad vibes hanging in any corners of my horizon.
Fifth Month
Allow me to fast forward. We had our first fight. I suspected that he was messaging his exes behind my back, and I was perfectly right about it all along. I saw his phone by accident while he was sending a message and twas flooded with all the messages when we had our good nights after a long day. All along, I thought everything was okay about us. That was all I thought. My Bad. I confronted him, and asked him if he still has feelings for them.