Honestly,
I like being best friends with people of the opposite sex who are taken. I feel safe around them. They won’t ever fall for me, and I would never fall for them. And I’m not a home wrecker so I wouldn’t do anything to take them away from their significant other. We’d all like a best friend of the opposite sex that we’d never develop feelings for.
The only problem is that their significant other can get in the way. They can get jealous of you, which I guess is natural. But just for once, I want to be best friends with someone taken and be able to keep that friendship without any interference.
