I saw that post about the friend Mary, of course, but I've been thinking about this for a while.

I don't think the problem is being friends with women, it's the other issues that usually accompany it. Being outright excluded from a group outing is a red flag. Your partner not taking your side is a red flag. Not being a vital part of their friend group is a red flag. Insisting on being alone at inappropriate times is a red flag.

All of these things would be red flags even if no women were involved. In fact, I'd assume "the boys" were just going to strip clubs and gossiping about each other's sisters and making misogynistic jokes.

Keeping a man away from other women won't stop him from being tempted. I've heard of guys cheating on their lunch break. So I don't think women being around is inappropriate in and of itself.