Oh my, . . . in my opinion there is something REALLY important to mention here:
Daisy WAS in fact drunk once upon a time, and that time reveals exactly why she never planned to get drunk again. Just listen to Jordan talk about it:
The day before the wedding [Tom] gave [Daisy] a string of pearls valued at three hundred and fifty thousand dollars.
I was a bridesmaid. I came into her room half an hour before the bridal dinner, and found her lying on her bed . . . as drunk as a monkey. She had a bottle of Sauterne in one hand and a letter in the other.
"'Gratulate me," she muttered. "Never had a drink before, but oh how I do enjoy it."
"What's the matter, Daisy?"
I was scared, I can tell you; I'd never seen a girl like that before.
"Here, deares'." She groped around in a waste-basket she had with her on the bed and pulled out the string of pearls. "Take 'em down-stairs and give 'em back to whoever they belong to. Tell 'em all Daisy's change' her mine. Say: 'Daisy's change' her mine!'"
She began to cry--she cried and cried. . . . She wouldn't let go of the letter. She took it in the tub with her and squeezed it up into a wet ball, and only let me leave it in the soap-dish when she saw that it was coming to pieces like snow.
But she didn't say another word. . . . When we walked out of the room, the pearls were around her neck and the incident was over. Next day at five o'clock she married Tom Buchanan without so much as a shiver. (Fitzgerald 77-78)
The letter, of course, was from Gatsby. Gatsby who was desperately working on the status and money he needed to obtain Daisy. Gatsby whose ship had sailed a little too late. Gatsby who was still desperately in love with (or obsessed with) Daisy. The same Gatsby who now sits and stares at the green light at the end of her dock.
When Daisy gets drunk, she falls for Gatsby. Period. She is married now. Married to a brute. Daisy cannot allow that to happen again, that flood of emotion pushing her to throw her safety, her security, away. She must think with her head, not her heart. Alcohol, as always, removes all inhibitions. Alcohol destroys even the most solid of social barriers: the barriers of the "old rich."
Alcohol will always force Daisy to reveal too much.
Daisy, then, avoids alcohol for that reason. Hmmm, perhaps Daisy is smarter than I usually give her credit for.
You can find the answer to this in Chapter 4, towards the end of the chapter. In general, the idea is that Daisy can keep a better reputation than she would be able to if she drank like everyone else did.
For one thing, if you are the only sober one among a bunch of drinkers, you can "hold your tongue." In other words, she didn't end up saying stupid stuff while drunk.
The other thing that Nick says is that if you are going to have any "irregularities" (he seems to mean if you are going to have an affair), you can do it while everyone else is too drunk to notice.
He suspects she did have affairs -- something in her voice -- but did not know because she didn't drink and therefore had the advantages I just mentioned.