Even having a digestive tract with two ends is a complicated arrangement. Animals like jellyfish, corals and sponges, who were the earliest evolved animals, have a large body cavity with just one opening leading to the outside environment. They eat, shit and have sex using the one hole.
Sponges reproduce sexually by shooting their sperm into the water. (Copulation is a land animal invention, developed to cope with the lack of a hospitable, moist environment and free transport provided by the ocean. In light of this, a uterus can be thought of as a mini, internal, ocean substitute.) Each sponge hopes (metaphorically, sponges have no brain) that its drifting sperm will be 'eaten' by another sponge. If one's vagina doubles as one's mouth, there is a danger that the sperm will not be used to make baby sponges, and instead be digested as a nutritious food source. Happily, sponge cells are capable of telling the difference between food and the sperm of other sponges.