Thirty years after Voldemort's defeat, at a party, Harry reveals that his ahem 'First time' wasn't great. Fleur is appalled. So she does what any good friend (in her opinion) would do. She travels back in time to give him the experience he deserves.but it's closed loop time-travel as already established in canon. So that means that Harry's actual first time had always been with Fleur. But he wasn't lying about his perceived first-time being bad, so even though they've both caught feelings for one another, Fleur has to obliviate Harry (he agrees) at least up until the point of her traveling back in time so that reality doesn't collapse. Harry's bad 'first-time' is bad because it's not with Fleur, and he can tell that something just isn't quite right. Fleur returns to her time just after she left, and dispells the obliviate. Happily ever after.