I don't think it's unreasonable to think your spouse shouldn't have an opposite gendered friend with whom they spend a lot of alone time. It's not just about trusting your spouse, it's also about trusting the friend. Couple friends and friends groups are great, not sure why my husband would need to spend a lot of time alone with another lady. There are plenty of women I trust him to be alone with, but they're all also my friend, and usually their partner is a friend as well.

Last time my husband had a female friend who was not mutual she ended up sending pictures of herself drinking in the shower to him. She was also in a relationship at the time, so he thought she was just a platonic friend. I trust him, I don't blindly trust people I don't know.