It's wrong to impregnate a minor.
Harry meets Sally at a dance club and takes her home with him in spite of the fact she's 17, and he's 18.
Harry wants to get laid.
Harry decides to use protection while having sex with Sally.
Harry decides not to use protection while having sex with Sally.
Harry ends up spending a fun night with Sally and looks back on that night fondly.
Harry ends up getting arrested after Sally is discovered to be pregnant and claims he's the father.