In modern times he would have had a surrogate. Abraham's wife initiated it.
True. Not claiming Abraham behaved badly, by the standards of his time.
You can read all through the OT, and not find a trace of the notion that a married man is obligated to be sexually faithful to his wife. Adultery in the OT is defined as sex with a married woman other than your wife. NOT as a married man having sex with an unmarried woman.
The NT is, of course, quite different in this regard.