How do I convince my parents to let me solo travel somewhere?

I have been talking to this very sweet girl for a while and we decided it should be best for us to actually meet. I am 18 years old and I have travelled without parents before, even when travelling with someone I am usually the one that takes care of almost everything regarding the trip, so responsibility should not really be a concern. My parents are very homophobic and I dont think telling them that I simply want to go somewhere randomly will satisfy their curiosity. I was thinking of bringing up some volunteering opportunity in the place I plan to go but once again my mother is not the kind of person to let me go somewhere just for the sake of the trip, always must have productive reasons behind. I have tried telling my mother of my preferences in the past so telling her that I am going to see a girl will not work..I need ideas and advice, please.