Some people are wired differently.
I've found that liberal people have a more relaxed, take-it-or-leave-it attitude toward sex. They like it, but it's just one pleasurable thing and it doesn't come with all the psychological extras that others get with sex. So they can't understand why more conservative people are so uptight about sex and want to put restrictions on things like pornography, gay marriage, etc... If it feels good, then what's the big deal?
More conservative people are much more uptight about sex and with good reason. Along with sex comes issues of control, power, lust domination, original sin and every other messed up thing in the human psyche. No wonder they want to put restrictions on sexual behavior...you would too. Conservative people are actually fifty times hornier than everyone else, contrary to popular belief. Even the horniest dog liberal like Jack Kennedy couldn't keep up with someone who was wired more conservatively.
I bet you're a more conservative guy, which is why sex to you is the culmination of the physical, the psychological, the emotional and the spiritual. Your girl just isn't wired that way. To her it's fine, but it's just sex.
So go find some sexually repressed lady and she'll be a wildcat. She'll be trying to suck the pants off of you.