I seriously doubt the ability of fairies to function as condoms.
I feel like she'd slip off and fall, getting trapped inside Wendy's uterus.
Peter and Wendy would be unable to figure out a way to get her out, so they'd just keep her there and keep fucking.
Eventually, Peter would finish inside Wendy and they'd both fall asleep, completely forgetting in the morning about the little fairy still struggling inside the now pregnant Wendy.
Fairies make far better onaholes than condoms, in my opinion.