That's a common misconception. No, Meyrin did NOT steal Athrun away from Cagalli. Cagalli broke up with him in favor of trying to lead Orb and helping it recover.... again. However at the end of the series Meryin and Athrun stay behind on Orb. As for who Athrun ends up with was never resolved. The Gundam Seed movie was supposed to resolve a lot of lose ends (including this one), but ever since it was canceled, we'll never know. What we did get was a few hints that Athrun returns to PLANT, with Meryin as an Orb Attaches to assist Lacus in her rule as the new PM of PLANT. There was even a character profile drawn up of her as a 20 year old, but I lost it after my HD crashed several years ago, but I never could find it again.
Sadly Meyrin has unfairly earned the ire of a lot of Cagalli fans.