What can I do to deny being baptized, or should I just get over it take a dip in the creek and pretend I'm a Baptist?

About baptism, I always say: water-magic is just as powerful as any other kind of magic - and - skin is waterproof. Getting baptised seems like it’s the easiest thing to do in your circumstances.
Personally, though, I wouldn’t do it. I think I would prefer the fight to being manipulated into doing something I don’t want to do, but each to their own. You’d also be living a lie for an (possibly extended) period of time - that won’t be great for your mental health, but only you can decide if that’s worth it or not.
As for how to get out of it - You don’t have many options Be honest with them doesn’t look like it’s something that you want to do, but if you aren’t at least 100% honest with your g/f, you are basing your relationship on something she may well resent you for later. For her family, there is always the fallback ‘I’m a methodist and I’m not going to change’ lie - stick to your guns and power through.