Monsters have a heat for females it's the only time they can get pregnant and for males it's the time they go into a 'fuck crazy daze' and go to fuck any female they don't share blood with, monster can still have sex for pleasure outside their heat but no kids will be made.
When a male teenage human falls down Toriel takes him in but mistakes him for being in heat after seeing his hard cock and knows how it must be hurting him not to go out and mate so allows him to use her body.
