Yes, yes, I absolutely agree with burundi. However, there might be a way around that, if (and ONLY IF) you have someone very close to you, like a sibling, who can broker the deal. That way, the sibling is prepared to field questions from friends and relatives who might be wondering what to give you and your intended as a wedding gift. Depending upon the person asking, the sibling could then, ever so tactfully, have a conversation like the following:
Great Uncle Moneybags: What do you think I should get for Cat’s wedding? I was thinking of just giving cash, because so many people enjoy having a little extra spending money on the honeymoon.
Sibling: What a marvelous idea! I know that Cat is feeling just a wee tad of stress about all the costs of the reception, and cash would certainly be thoughtful. Say, now that we’re talking about the reception, it occurs to me that instead of the honeymoon, perhaps a bit of assistance with the bar would also be a lovely gesture.
Or, if you don’t have a Great Uncle Moneybags who is likely to ask others about gift ideas, said sibling could also organize other siblings, or cousins, or friends, or the wedding party, to go in on the bar as a group gift, provided of course that the result is at the same level of another group gift of china or linens or whatever would have been. This works better if your set tends toward group gifts in the first place, and the organizer should have a good sense of a reasonable amount to propose to the group. Then you should be sure to make a light-hearted yet sincere toast to the group (or the uncle) at the reception, acknowledging the most generous and thoughtful contribution to making your special day so perfect.
The key here is that another person is doing the proposing of the bar cost as a gift, and not you. The person should be tactful enough not to frame this as “Cat says cash for the booze would be a lot more useful than fish forks” but rather something more along the lines of “I heard from Cat that they are having a devil of a time with the lack of a proper bar at their venue, so I was thinking that a nice gift from all of us might be to take over the arrangements and chip in for the cost. Plus then we don’t have to lug fish forks all the way to France.” You have to be able to trust this person with your life, or at the very least, your social reputation, to manage this delicate negotiation, but it can be done.