Needing to fantasize to enjoy sex

So I grew up in a Christian household (somewhat) - my father was the son of a methodist reverend and although dad himself was not a big churchgoer, he ensured that my sister and I attended until we were 10 years old. I'm currently not a firm believer in any religion, yet I just can't seem to discard the issue of premarital sex. It's to the point that I usually have to envision that my partner and I are characters from a movie while we're making love, for me to loose my inhibition and just relax. Mind you, we've been in a relationship for about 6 years now (we're 26), and he is completely committed to me. Anytime that we've had relationship issues in the past, it's only ever been me who states/threatens to call it quits. Is this situation unique to me? Does anyone have any advice? Regarding why we haven't legally married yet, he believes in marriage on a spiritual level, and considers himself married to me already (as do I), but finances never allowed a formal ceremony to happen.