Complex sentence: Samantha's parents try to talk to her about her behavior, and Samantha eventually tells them of her and Lonnie's relationship; they put it down as "a phase", not believing and accepting that Samantha is a lesbian.
Split into simpler sentences: Samantha's parents tried to talk to her about her behavior, and eventually discovered her relationship with Lonnie. They put it down as "a phase", not believing or accepting that Samantha is a lesbian.