You initiate the scene by first getting a hand job in the bathtub and should you have the requirements your character will ask about sex, if I remember correctly you need to do the basement titfuck 3 times to get this option unlocked. Naturally as this is a fresh update with no bug fixes so far we can assume that there could be a bug involved so worse case scenario you'll have to wait for Doc to fix it. Use my guide for any more info you two need.
And for
nico12 my guide had marked the first time sex with Martha as just "
Sex" I'll rename it to "
First Time Sex" this way you'll find it easier.