I have seen such activity, interestingly it was among waterfowl. Ducks specifically. Ducks do not strike me as very intelligent - they have a strong libido in the springtime but don't have the smarts to tell the difference between male and female ducks. The male ducks basically jump on anything that looks like a duck, regardless of gender, and hope for the best. I should note that these trysts look very violent, with the penetrating male jabbing the neck of his male or female partner with his beak very violently. If I had to compare it to human behavior, I'd say these ducks are rapists. Yet this is "natural." This is why it is so silly to use animal behavior as justification for human behavior. Lots of nasty behaviors come naturally to animals - does that mean people should emulate them?