I do not believe that rape shield laws prevents defendants from having a fair trial because I do not believe that a woman's sexual history has anything to do with whether or not she can be raped.
To think about this, let us look at the definition of rape. As the "wests-law-encyclopedia" link below says,
Historically, rape was defined as unlawful sexual intercourse with a woman against her will. The essential elements of the crime were sexual penetration, force, and lack of consent.
Obviously, a woman's past sexual history has nothing to do with whether sexual penetration can occur or with whether force was used. So then the question is whether her past history can tell us anything about whether she consented to sex.
The only way a woman's sexual history could matter is if it meant that she would always consent to sex. This is absurd. The fact that a woman has had sex with any given number of men says nothing about whether she wants to have sex with a given man at a given time. It is simply old-fashioned to say that a woman is either a "madonna," having sex with only her husband, or a "whore," who is fair game for any man at any time.
A woman's sexual history cannot possibly bear on whether she consented to have sex with the accused rapist during the incident in question. Therefore, rape shield laws do not prevent a defendant from having a fair trial.