Yowza! I'd say that behavior is called "adultery".
http://thenarrowpath.com/ta_divorce.php
You may call it that all you want, but the fact remains that the Bible has a different definition of adultery - already referenced the citation above. Adultery in the Bible is strictly and *expressly* defined as “**marrying** someone not your spouse while still married to another” or “marrying someone not your spouse while *they* are still married to another.”
The Bible does NOT prohibit concubinage or ‘sleeping around’. Which, given the era and inheritance laws of the time, does make sense. It doesn’t say it’s a good idea (in fact, in 2nd Samuel it implies that it’s a bad idea and can ruin your life) but it’s not forbidden nor is it a sin.