Guy here who has always had lots of female-friends.
The thing about “female-friend | male-friend” is that fact that these friendships tend to be one-sided. And what I mean by this is almost always the woman is the person who is in control of the two people being more than friends. I really hate stereotyping but you are kidding yourself if you think nearly every man would not sleep with his “friends” given the chance.
Women and men can be friends. But these friendships can become a problem due to the way that men (usually) think.
That being said, it’s not a red flag until he as an individual makes it one. Yellow flag yes, but not a red flag.