Odd Girl Out by Ann Bannon

Laura Landon is a sheltered freshman at a fictional university in a Midwestern town. Intensely shy and introverted, she is drawn to the president of the Student Union, Beth Cullison. Beth is outgoing and friendly, experienced socially (with men, particularly) but feels a void in her life. She doesn't understand how the other girls are so fulfilled by the men in their lives, despite having tried. Every time she allows herself to be intimate with one, she breaks it off out of disappointment.