A guy who is boyfriend material is respectful of his ex girlfriend, which shows women he has moved on and isn't going to carry too much extra baggage into the relationship.
One of the most unattractive things you can do as a potential boyfriend is to be disrespectful of your exes. Sure, it likely ended badly and it's a touchy subject. But, displaying raw feelings to a girl you're dating only makes you look immature. After all, she could one day be your 'ex' and wouldn't want you calling her names or spilling her deepest secrets. Plus, if you're truly over her, she shouldn't be a main topic of conversation anyway. To show you are an adult who can maintain and survive after intimate relationships, show a little respect towards your ex.
- Rated #3 in 7 Qualities Women Look for in a Man