Assuming that the guy or girl telling you this is past the normal 'first kiss' age, like in their 20s. Would you want to know beforehand or after? I feel like this might be traumatizing and worrisome for guys- they would think she'd become clingy and psychotic. And it would be a 'so cute!!!' sort of thing for girls because...well because it's cute.
Most Helpful Guy
One girl did tell me I was her first kiss. It didn't freak me out in the slightest. When we stopped, I told her that no matter what happened, I would remember the honor of being her first kiss for the rest of my life. She replied, "Me too." I was right, it's been twenty five years and still counting and I still remember it with delight.1