It's rude to masturbate on someone else's property.
After a heavy petting session, Lacy's boyfriend disappears to visit his other friends leaving Lacy wound up at the party.
Lacy wants her urges to go away.
Lacy gets something to drink and steps outside to distract herself from her current state of mind.
Lacy walks into the bathroom and touches herself until she's satisfied.
Lacy meets other people with common interests outside.
Someone hears Lacy and reports her to the host who removes her from the party.