You can't control what your partner does. You can control what you do. If your partner has friends of the opposite sex, and you don't like it, you can either demand that your partner put a stop to it, or learn to trust your partner as you should.
If your partner has a Platonic friend, and you draw first blood in the relationship by refusing to trust your partner, then you have nobody to blame but yourself when you eventually do become a cuckold. You drove the wedge.
And if your fears were well-founded, picking a pre-emptive fight probably wouldn't have saved the marriage anyway.
Well said.