The lifestyle website HelloGiggles describes how the hashtag started:
In 2010, Duval tweeted: “#itaintrape if u naked of Yo Twitter profile pic”, “#itaintrape if I’m paying child support”, “Lmao RT @MrCashFanatic: @lilduval #itaintrape if she doesn’t remember it.”
From there, his followers took the hashtag and ran with it.
Following Duval’s lead, Twitter began referring to rape in a myriad of disgusting ways: as requisite payback for money spent on a date, as some sort of inevitable result of intoxication on the victim’s part, or as the consequence of the victim’s date initiation.
Two of the mentioned tweets are still online (here, here), one has been deleted.
If you search for the tag in a time frame when it first started, most of the tweets will be tweets using the tag seriously. Even if you search before 2015, a majority of the tweets are still using the tag as intended. The backlash against it didn't start until later.