> Most people would probably be just fine with their spouse having another best friend of the same gender, but definitely not of the opposite.

It seems to me that people to whom this makes a difference are both extremely insecure in their partner’s loyalty and extremely secure in their partner’s exclusive heterosexuality.

Take away the former and the concern disappears, take away the latter and the concern remains without the gender disparity.

It's not about the partner's loyalty. Loyalty has very little to do with emotions and works at a higher level. If someone spends a lot of quality time with an individual whilst their home life is stressful (kids etc) then they will almost inevitably develop an attraction to them (or to the idealized version that one has of someone one doesn't have to live with). Meanwhile at home feelings of being in-love have faded to loving and the day-to-day realities are there and obvious. Then it is game over and loyalty is fairly unlikely to prevent things happening.

Better to spend quality time with your spouse and re-enforce the bond the two of you have for each other.