It never ceases to amaze me People convince themselves that religious differences won't be a big deal in their relationship. This article does a nice job in looking at why issues arise in interfaith marriages. Almost all of this applies to marriages between Christians of different fellowships.
Life in a fallen world, and marriage that involves two sinners, is hard enough when that couple shares a common faith and confession. Things become exponentially more difficult when faith itself becomes a source of friction and strife.
Thanks to Hicham Chehab for posting this.