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When it's cold outside and you can wear boots and scarves and layers and look sexy
*It's cold outside

Girl: "It's sexy time weather. Can wear my boots today!"
by guerillagirl January 10, 2011
When it is raining/thunderstorming/cold outside, the perfect time to lie in bed with someone all day
*Raining outside

Girl: It's sexy time weather!
by guerillagirl December 31, 2010