NOT HUMANS - Not Exactly Anyway
It might not "seem" right because we love our dogs/pets so much and they have such unique and different personalities that we often think of them as being "human". Now I know that this can be contoversial - especially amongst fellow dog lovers, but when dogs mate, there is no love. They have no attachment to each other and the physical act is in no way romantic love. It is pure instinct. No feelings, no emotional fallout. A pure physical, natural response by the bitch and sire.
So don't think of them as two people in love. A caring and devoted husband and soon to be proud father. A mom who is being violated, etc., etc. They don't really process information that way. A female in heat is looking to get pregnant. Natures way of continuing the species. A male dog around a bitch in heat just wants to impregnate her - instinctively. A female bred with two separate dogs doesn't "have any problem with it". At least that's what "the dogs tell me"! :D