How are rape shield laws not unconstitutional?

Because how are her past relationships relevant to the case? The question isn't whether she's said "yes" before, it's whether she said "no" now. And the fact that she's said yes a lot before, doesn't mean that she can't say no now. Or that it's less believable. A woman with many sexual partners has just as much of a right to give and take away consent as a virgin. Her sexual past shouldn't matter.