Girls got excited while riding stallions, so it's not surprising that when they saw those guys they jumped on them. Well, their chosen pose is exactly what I pointed out in the previous sentence. It has always been a mystery why so many girls love horses, actually this video partially answers this question.
Loved her sister's reaction the moment her brother pulled out his dick. What did she expect? Climbing on her haunches with her legs spread with her panties hanging out in front of him and she thought he'd get over it?