So I found this fic a few months ago...
-Dean was drunk and raped Sam in a bar
-Sam was sober that night
-When Sam told Dean that he raped Sam, Dean was appalled and didn't believe it
-One day, Sam finds out he's pregnant
-Sam was a virgin
-Bobby urges Sam to tell Dean that Dean raped Sam...
-I am 99.99999% sure this was on fanfiction.net
FOUND: https://www.fanfiction.net/s/7965009/1/Out-With-the-Truth (its a series tho)