Not all male best friends are scum. I'm close with my best friends wives. We are a very tight knit group of friends that is more like family (like we're "Uncles" to their kids). No funny business there because I have too much to lose. I love my friends and social circle.

Not everyone thinks this way. If there's some random dude your wife/gf is friends with, you have to ask why they never got together in the first place, and if she's keeping him around for the ego boost when her actual relationships hit a snag. I've been that friend too and I can tell you I didn't value those friendships nearly enough to keep me from making moves when I saw an opening. The flirting was actually one of the perks of the friendship.

Also watched a friend in another group of buddies I have get banished for dating the recently divorced ex of another friend in the group. They were "best friends" before her divorce. They did a lot of flirting with each other before she and our other friend divorced.

TLDR - You have every right to be uncomfortable with the male best friend. Get a female best friend, see what your wife/gf thinks about it.