Tell her you can't picture her raising a child, therefore could never ever marry her, but you would very much like to be intimate with her - just for fun like  .  Wink just like that.

She won't just break up with you; she'll put such a a curse on you . . .

thanks to all for your comment. In NL we have the good the bad and the ugly. Na wao Boy dey vex.

If truly u've not slept with each other,i think it's gonna be a lot easier but she's still gonna be depressed.Just go ahead and tell her,watch her reaction and you take it up from there.