After the whole Grindalwald thing Credence miraculously survives and Graves takes him under his wing. Along with teaching Credence magic, Graves decides Credence should also get a thorough education on how to please the fairer sex. The obvious choice of woman to help Graves accomplish this is someone Credence likes and trusts aka a bewildered and probably very flustered, but secretly very eager Tina.

Credence can get involved for a hands on demonstration, or sit back and take it all in. I wouldn't mind any Graves/Credence overtones or a happy threesome either.