My girl chickens out... what to do?

Don't engage with the "big talk" and instead tone down the rhetoric to a more realistic discussion.

She appears to like the concept of a sexy adventure with others, but isn't ready to fully engage another couple or single, so accept and respect where she is in the process and go slower; incremental baby steps.

When she becomes gung-ho, let her verbalize her fantasy. Simply acknowledge that you like the idea too, but suggest that if you ever decide to meet anyone in the lifestyle, then all should agree in advance that the initial get together is simply a no-pressure meet n' greet at a vanilla location. Your lady might find that concept enticing, and not back out considering there's no expectations. And the ensuing sexual tension (without expectations) might help her get past the hump (pun intended) and then more gradually take it to the next level; perhaps in-person voyeurism or same room, no swap, soft exchanges. Eventually, someday, you two might find yourselves betrothed in an all-out sexual frenzy with other couples (plural).

Meanwhile, take the pressure off and go forward leisurely, and enjoy the process.

I agree with the notion that she just may not be into it, however, she should not play this game with you. You're obviously ready to start exploring, otherwise these conversations wouldn't had take place, and she's stringing you along (if she's drunk when she talks big, all bets are off and you should know your partner better and know that she's just BS'ing you). Be very forthright with her and give her an ultimatum - yes, they're necessary sometimes: Either stop with the big talk and if she continuous let her know that whether she goes thru with it or not the next time, you intend on caring on and at the very least meet the couples and start exploring on your own -and follow thru. The other answers are what husbands know their wives want to read, believe me, they'd be just as frustrated if they were going thru it. No one deserves to be played with.

If you really know your mate and hopefully have a honest relationship you can resolve this question between you. Maybe she is just really shy. Like a guy that didn't date much in school not because he didn't like girls but chickened out. And alcohol won't make you do something you really would not do like gay sex if that's something you just would not do.