Sit them down, and just tell them.
"I'm a lesbian".
Just tell them that.
Then tell them that you are still their daughter. A few seconds ago, you were their daughter. A few seconds later, you are still their daughter. And you will continue being their daughter.
Nothing can change that.
And just let them have at it.
You can tell them the truth now, and at worst be damned for it. Or you can tell them later, and be damned for it.
If you ever wish to get married to your partner, granted your government permits you the right to, do you not want your parents to be there? Give them time to adjust, or atleast as much of it as you can.
Cheated the way from fringe to elite. Clique of stylists, rounded illogic skipping a beat to a dead cert. By lheaving charges and bursting the abscess, with a forked toungue, bloated with courage and spewing self-importance. Drop your sights, aim lower, leave umblemished those with real power.