Basically, Abrahamic religions were designed at a time when the patriarchy was even more powerful than it is today (although the government seems to want to bring it back to those "good old days" presently). The Bible was composed at a time when women were chattel, and had no power to speak of. They were barely considered human. Their function was mainly to be breeding vessels. They certainly could not have more than one male sex partner. So, that's the bottom line.
However, the modern church also condemns men having more than one wife. And bigamy is illegal. So, having a wife and a gf is not allowed. Period. Some liberal Christians try to get around this by saying Jesus's message of love for all overrides the idea of monogamy, but that's a stretch, imo.
However, the modern church also condemns men having more than one wife. And bigamy is illegal. So, having a wife and a gf is not allowed. Period. Some liberal Christians try to get around this by saying Jesus's message of love for all overrides the idea of monogamy, but that's a stretch, imo.