Huh?
Because you are spreading YOUR genes further?
I thought that having LESS of a selection is what is supposed to be bad for evolution...
Not exactly, it's because you are spreading your multiple genes into a confined area increasing the odds of inbreeding.
Take a not so hypothetical, a man has 4 children with 4 women. What are the odds of half siblings mating, especially if the children don't know their father?
A few more generations and the odds get really bad.