from what i have read semen can stay alive for 3-4 hours when in contact with the air. and when in enters the vagina it can stay alive for up to 7 days becuase it is protected. so IMO i think that there is a possiblity of getting a pregnent.
Direct air kills sperm eventually. There is a grace period with sperm on live and inanimate objects. Sperm STILL DRYING on underwear and towels etc could get her pregnant still, but your sperm would have to be abnormally potent and active. Once it turns white and flaky, youre okay. Your finger however is warm enough to keep them alive awhile longer, but still slowly dying off. My suggestion is stop having sex until you are more informed about it (and can practice proper handwashing habits.) When you truly are READY for sex, you dont 'finger' around the bush, you just jump into it. (if you know what I mean....)
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Once it turns white and flaky, youre okay. Your finger however is warm enough to keep them alive awhile longer, but still slowly dying off. My 
