If it were me, I'd do it. You're not marrying them. You're playing some gigs, spreading your wings, pushing at your comfort zones, meeting new musicians, gaining a new experience, and I could go on and on and on. You also have no clue how you'll feel once you're out there gigging with these people. If you continue to despise the music and aren't having fun, in 6 months you can tell them to start looking for someone else.
The only thing I WOULDN'T do is express any of your dislike of the music to them. There's no need for that, whatsoever.
And RE the music, I too was never a country fan. New band I'm in pays excellently (it was my motivator) but also does a bunch of country, and hip hop (which I hated even more). What I learned is I learn to love playing everything by putting my mind in the right place. I commit to playing everything I play with all the passion in the world, and it's turning into an awesome experience. I'm loving playing with these guys. But that's not even what I really wanted to say - just went off for a minute

...
I hated country music until I started learning some songs. Listening to this song did less nothing for me. Learning it note for note gave me a whole new perspective. This bass player kicks ass IMO. He's tasteful, and I like the way his basslines evolve and intensify as the song moves along. Have a good listen to the entire song (focusing on the bass, of course), and I challenge you to then tell me you don't like what this dude is doing.
Also, as much as this song did nothing for me too - it's become impossible for me to play without smiling. Especially when I see the crowd reaction. Song actually starts a 1:10:
That's some seriously fun bass stuff to play!
Join the band.
Quit in 6 months if you have to.