>>149874132
Repressed lesbianism. She goes after the blandest boy imaginable because she's coping with the fact she likes girls. Marcie is the same. This is actually pretty common with young girls discovering they are lesbians for the first time, where they both "crush" on the same boy as a proxy for their feelings for each other.
It's subtext that make Peanuts so much deeper and interesting than you'd expect on the surface, Schulz was ahead of his time.