For a couple of reasons. One, because she will never truly commit to you.

The bigger reason is because we as men are dominant. As men, we are naturally visual creatures with the desire to spread our around.

If we forsake our desires because some woman tells us that she'll leave if we don't, we are SUBMITTING to her.

The average woman would be a groupie for Brad Pitt or Drake or whoever the fuck without a second thought. Maybe we don't have their money or status, but that doesn't make us lesser men. If a woman will do something for one man, we must REQUIRE that she do it for us as well.