Why is it bad for religious Christian's to have a champagne toast at a wedding?

It's NOT bad in terms of The Bible to drink champagne (wine) in moderation. However, it may be bad in the sense that it may upset and cause problems with those who oppose the consumption of alcohol.

However, the Bible makes it clear that Jesus drank wine (Matthew 15:11; Luke 7:33-35) and approved of its moderate consumption (Matthew 15:11). On the other hand, he was critical of drunkenness (Luke 21:34, 12:42; Matthew 24:45-51).

The later writings of St. Paul (d. 64?) deal with alcohol in detail and are important to traditional Christian doctrine on the subject. He considered wine to be a creation of God and therefore inherently good (1 Timothy 4:4), recommended its use for medicinal purposes (1 Timothy 5:23), condemned drunkenness (1 Corinthians 3:16-17, 5:11, 6:10; Galatians 5:19-21; Romans 13:3) and recommended abstinence for those who could not control their drinking.