You'd probably want to focus on the B&G and get the background acceptably sharp so that acceptable detail is visible.
The easiest thing to do would be to focus on the B&G and press the DoF preview button, or take a few shots at decreasing aperture and select the one you're happiest with.
If you're shooting with an APS-C camera and moderately wide lenses I don't think you'll need to resort to extremely small apertures.
This^
I wouldn't want the church as sharp as the couple, they're still the subject. Remember that there is only ever a single plane of focus. Some people seem to think that the DoF is a distance within which 'things are in focus' this is abetted by landscape shooters going on about hyperfocal focussing :nono:.
The fact is that the plane of focus is a single distance, dependant on the usual DoF variables, everything before and after that distance is OoF to some degree, whether that's slightly OoF so that it's acceptably sharp or completely OoF so that it's creamy Bokeh and all points in between.
Still the fact remains, if a subject is shot against a busy background, with no help from DoF or lighting, the subject will disappear into the background - it'll cease to be a portrait. This will obviously be made worse if the background colours are more dominant than the subject colours.
Think about a footballer wearing a black or green or grey kit against a stand full of red seats and fans wearing red and white. With a really shallow DoF you'd have a footballer, with enough hint of the crowd behind to build context. With a really deep DoF the footballer will almost disappear, you'll have a photo of the crowd.
As with most things photographic - it starts with 'what is it you're taking a picture of?' and Why?