Sometimes, you just aren't ready yet. No shame in that, man. But don't force yourself either, that's not a cool look, bruh.
Waiting until marriage is a noble goal. If your're committed enough to marry a girl, you're mature enough for sex. Still, if the time is right - you're ready and they're ready, why hold the relationship back? You can always get married later. Don't ruin a relationship by putting conditions on your partner, demanding or withholding sex, you'll regret it.