The answer in my opinion comes from a few reasons.Take a step back with me for a moment to 6/30. This scene was very hot but it introduces another minor plot hole:
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Both Kirito and Asuna know he's a quickshot, so you'd think we would have heard of this item before now. Why haven't they already been using it? Is it a rare or expensive item? Maybe Kirito can't use it for some reason? I guess it's as simple as "there wouldn't be a story if Fujino thought of that" but I was hoping there's a better excuse.
1. Kirito doesn't know he's bad in bed with Asuna
2. Similar #1 Asuna fails to communicate anything about this stuff to Kirito at all
3. Kirito and Asuna are the type of people who don't expose themselves to places like brothels or more sexual deviant related things. This also probably means they never considered a male stimulant like viagra, or the FOG durability potion equivalent.
4. Sinon and Suguha are more exposed and a part of this darker/naughty side of the world, so they know it exists compared to Kirito.
Tl;dr
Kirito/Asuna don't communicate and their initial relationship had them both as prudes.