The ocean is a big place. It's not easy for male dolphins to find females while they’re fertile.
When males do find a female, they tend to pile on, as you can see here.
Female dolphins can't avoid the amorous males.
But the females have a secret weapon.
All dolphins, whales, and porpoises have 'vaginal folds' in their vagina.
These fleshy folds can block the penis's path to the cervix, depending on how the female shifts her body position.
a) shape of harbour porpoise penis, b) shape of harbour porpoise vagina as 3D model and c) after dissection.
If the female does not want the male's sperm, she can shift her body to misalign the fit of the penis inside her.
Image from Orbach et al., 2020
a) Common bottlenose dolphins with more complex vaginas and a vaginal fold that likely blocks the penis.
c) Short-beaked common dolphins with relatively simple vaginas.
Image from Orbach et al., 2017
References and Further Reading