I have two generations, one after the other, where neither couple seem to have had marriage entries. In both cases, their childrens baptisms were registered. The experts on here hunted high and low without result. So am stuck at 1740 and likely to remain so.
The usual answer is the marrying couple wouldn't or couldn't afford to pay for the entries in the parish register. Yes, you had to pay a fee. The children were registered so as to establish their legal residency in the parish, in case later they had need to apply for parish relief.