To put it very bluntly, I think people who are uncomfortable with the thought of God giving David his master's wives to have sex with, are exhibiting redidual faulty thinking that goes contrary to creation and PROcreation. They might be unaware that their discomfort is because of a long ago departure from the Biblical mindset....knostics, then the church at Rome, then continuing down through protestant churches too, but it is just as much a departure from true faith and obedience to the creator.
So they try to redefine marriage to them being "friends?" "Roommates?"
Or like I said before more like the women being a servant in the home.....because they think (or feel) it would be wrong for the new king to marry the dead king's wives.