analog

If she REALLY means this, then there's another underlying reason why she won't have sex with you. Most likely she still does not feel comfortable enough with herself- either a self-image thing, or she's battling feelings of "is he the one". You can be pro-premarital sex and still think it's necessary to be sure you're with "the one" before having sex with them.

In my opinion, that's kind of self-defeating and pointless, since you're basically advertising yourself as "ok with sex before marriage" when you really mean "ok with sex once I'm sure the man I want to be with forever"... which corroborates her belief that "marriage" is a piece of paper. She could care less about the title, she wants to open herself up to "the one", not a title.

So, my guess is simply she's not yet sure you're "the one", and she is waiting for "the one", not the title of "Mrs. The One".

I would ask her, straight out, that since she's for premarital sex and considers marriage a piece of paper, if she doesn't consider you "the one". Ask her if that's why she won't have sex.

The fact that she doesn't even want to talk about it means she's fairly certain she doesn't want to do it... and knows that if she invites conversation, it will only make you ask more.