Is it okay if the groom's father is not the best man in the wedding? Should he still serve as a wedding usher, or should he be seated with his wife?
There is no rule that says the groom's father must be the best man; many grooms do choose dad to do the honor, but just as many select a brother or best friend. So of course it's okay! Having a dad stand up as groomsmen (or ushers) if they're not best man, but that doesn't mean the groom can't ask dad to do so. If the groom's father isn't part of the wedding party, he should sit in the first row with his wife. The role dad plays in the wedding is really a personal decision.
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