Okay, that puts an entirely different spin on things for me.

At our church, we often host grocery showers for new couples.

I thought that you meant the new couple was ASKING for people to bring the food.

It is never appropriate to ask people to give you things as gifts, but it is certainly fine, IMHO, for a church to celebrate a marriage with a potluck and a grocery shower.

I will also say that it is awkward when you have someone getting married in a church ceremony that has a connection to the church, and everyone and their uncle expects to be invited to the wedding. It puts the couple in an awkward position, particularly if they wanted to have a sit-down dinner and couldn't afford to feed the whole church... and when you put the announcement about the wedding in the church bulletin, the whole RSVP thing goes right out the window. You can't plan on a number, you would like your reception to be a more intimate event than you can 'control' when everybody shows up with their wild kids.