I really don't think you've thought this whole thing through, Rihanna.
The logistics and the realities of the situation.
Sure, it might sound nice as a throw away phrase but do you know what would have had to have happened for you to actually be the only girl in the world?
Have you thought about what would probably happen afterward?
We'll have to assume some sort of global plague as in the excellent comic book series Y: The Last Man because a world-wide gender-based massacre seems very unlikely and would have to itself be caused by some sort of transmittable psychosis (probably lab-engineered).
So there's been a plague, women and girl-children lying dead in the street, bereft men wailing and screaming as the more opportunistic take to looting and trying to establish power-based mini-societies in the aftermath of this civilisation-shaking development.
And you're the only girl in the world.
This can't end well for you, Rihanna.
At its most essential, there are only two outcomes: exile or death.
If your beloved manages to whisk you out of the major metropolitan areas and into hiding or even keep you hidden within a major city your situation will be tenuous at best.
What if there is a riot? Or a war? Or food runs out? Or your building catches on fire? What if there's an epidemic? A pandemic? What if your beloved goes out for food/water/information/to work and never comes back? What if he comes back but is followed by a mugger or a particularly cluey intelligence operative? What if you're seen? What if you start to doubt him? To think that it couldn't possibly be all the women? What if you go out to find out for yourself?
If you are captured for any reason you'll either end up in a laboratory with scientists frantically harvesting your eggs, artificially (or not so artificially) inseminating you and studying you to see why you haven't died yet; or, well, quite frankly you'll probably end up passed from man to man or the exclusive possession of the local powerhouse/warlord who is unlikely to be a gentle and considerate lover.
I know, Rihanna, you might think I'm being unfair to men saying that. I'm not saying that in the absence of a female population all men would resort immediately and with relish to rape but those who rise to power in uncertain and destabilised societies are usually determined and ruthless and probably not well stocked with empathy.
Even if they're intelligent enough to realise you're the only girl in the world they'd also know that you're no Eve, there's no way that in your life time you'd be able to replenish the world's depleted supply of women, so they might as well enjoy you whilst you're here.
So how in our current pre-lady-apocalypse society is the object of your affections supposed to make you feel like you're the only girl in the world?
Lock you in a windowless room and come back daily, whispering shaky, unconvincing reassurances and bringing dented canned goods and meat whose provenance you probably don't want to ask about?
Strapping you to a cold metal gurney under a bright light and talking about you but not to you?
Relentless physical abuse?
At the very best by saying you want to be made to feel like you're the only girl in the world you're suggesting that your lover may only be with you because the only other options are masturbation or reluctant homosexuality, not because you are an appealing and engaging individual in your own right.
It's you, loneliness or wang, Rihanna.
Doesn't sound so romantic now, does it?
Oh sure, I'm probably over-thinking this, reductio ad absurdum and all that.
I'm just saying, maybe you should spend a little more time on your lyrics.