What Mitzva does a regular girl have which would be in opposition to the Chiyuv of getting married?”
Check your halachic facts first. Only GUYS have a chiyuv to get married. And no one should get married whether they want to or not. That’s unfair to the unsuspecting spouse who is marrying them in good faith.
Just because the girl is not ready YET, does not automatically mean she NEVER wants to get married. Lots of girls are disillusioned by what they see. Their friends get married young, get tied down with kids AND supporting their husbands, and they just want to live their lives a bit before taking on these responsibilities. And maybe they don’t buy into the husband sitting and learning idea, but are afraid they will be censured for expressing their feelings. Maybe they are just NOT READY. Leave the girl alone for now. If you can get her to talk about why she is opposed to it for now, you might learn something important.
I had an acquaintance who for some reason confided in me that though she somewhat liked her chosson, she did not love him when she married him (though after the wedding she eventually came to love him). She felt guilty NOT about her feelings, but for having pretended to act and feel all the ways a kallah supposedly feels, according to her observation of her friends.
When she got together with her friends, she would “fake” giggle with them as they asked her about this or that, related to her chosson. She oohed and aahhhed over the things she got at her shower, and played the shower games as she had watched her friends play them before her. But it was all an act. She was not ready to be getting married, and felt none of the things she had expected to feel when contemplating marriage. Nonetheless, she talked herself into thinking she really wanted to get married, and her chosson was a nice enough guy. Had her family and friends not all been subtly pressuring her in different ways to be a kallah, she would never have gotten engaged at that point in time, though she has no real regrets after the fact. I found her confession made me sad.