My girlfriend's family wants me to be more formal.

My girlfriend's family is very formal unlike mine. I mean, my family's formal too but not as formal as my girlfriend's. They are Christians like us, however, their practices are killing me. Like, they encourage me to wear something at least presentable (polo and jeans), to attend in every church activities and to go to church every Sunday. I am a foreign man myself and they are expecting me to do all those. How should I handle this kind situation? It's kinda hard.