Are we not supposed to test everything, and keep what is good? I'm testing, thus far your theory is not standing up to testing.
David went into Bathsheba, but it did not
make a marriage. Absalom went into David's women, but it did not make marriages (Unless of course, God Himself nullified those marriages releasing the concubines from David). Your argument is
not that though as far as I can tell, it's that the mere sex act alone of a man with married women made them his women and then his widows.
Romans 7
"For the married woman is bound by law to her husband while he is living; but if her husband dies, she is released from the law concerning the husband. 3 So then, if while her husband is living she is joined to another man, she shall be called an adulteress; but if her husband dies, she is free from the law, so that she is not an adulteress though she is joined to another man."
If your argument is that sex = marriage, if it's with an unmarried woman, you have some scripture to make that claim.
But if you try to claim that sex with a married woman automatically transfers ownership to that man, do you not see how many problems and inconsistencies that causes?