Interesting topic!
Honestly, I am going to be very straight-forward with you. Yes, I agree with you; mates should not be cheating on each other and unfortunately, it's very common.
All I can say is dont fet your hopes up! Especially if you picked your mate when at the MateCenter. I would highly suggest focusing on Roleplaying and spending time with friends, because in doing so, you'll know who you can and cannot trust.
NOTE: No, I am not saying don't get a mate. There's nothing wrong with it. I am merely stating that its difficult to maintain a relationship without trust.
So, yeah, thats really all I have to say. Some mates are loyal, others just want their mate online and when they're off, they find one that is on. The cycle just keeps repeating.