crustsaway I can't give you a more convincing answer than "because sometimes it feels right".
I had been religious about not flirting with someone I hadn't met, when doing OLD, rebuffed it in others - either ignoring or telling them to go easy.
With current man, I looked back over our initial online conversation and was genuinely shocked how quickly our convo had turned sexual (verbal, no cockshot!)
Just... a big verbal spark.
FWIW, we found out the day before we met that he was being deployed (forces). We had 2 fantastic dates with crazy sparks flying, but both expressed a wish not to have sex because we didn't want to be intimate before a 3 month absence, as the absence could end things and we didn't want a one night stand. Though we both said we'd had those and made no moral judgement. Just: not what we were looking for.
You can go 0-60 online because it feels right, and slow down again!
But 99% of the time, even if the online stuff is working for you, I think it's a bad idea to get sexual before you meet!
I told him it was going to be excruciating if one of us didn't like the other in person. He just said "maybe we won't, but I'll still look bank and think this has been one hell of an amazing week".
Sorry for the hijack.
Just: context is all.