I\'m only a beer teetotaller...

The term "Teetotaller" refers to a person who does not consume any alcoholic drinks.

So in the sentence above, the speaker is saying he drinks champagne, but does not drink beer.

Others may have once thought that this person does not drink any alcohol because they never see him drinking any beer at social gathering, but this one particular occasion they saw him drinking champagne, so they might have raised the question regarding if he's a teetotaller.

To extend the meaning, he may also try to emphasize that he only drinks expensive alcoholic beverage (Champagne, for example), and not cheap beverage (beer in this case), as a social status.

A teetotaller is a preson abstains from drinking alcholic beverages.

This quote from George Bernard Shaw in Proserpine. Candida, act 3.

means it give champagne a higher regard than beer. Even those who think drinking alcholo do not drink beer, they will consider champagne not really a alcholoic drink. Or a person give himself a break or an excuse to drink.