Do guys really think they have a chance with their female friends, even if they’re in a relationship?

Is it true that guys always think they have a chance with their female friends, even if the guy is in a relationship? My boyfriend told me that it's just 'guy nature' for men to think this way about their female friends, and that’s why he doesn’t have any close female friends—because he believes he might think the same. Is this a common mindset for guys, or is it just him?

I am truly hurt with his statement and thought process