Its actually one of the most common kinks in the world and has ties to early human psychology. Basically, sperm competition for the same spouse is something instinctual. If a caveman saw his woman getting plowed, his first reactions would be to 1. Kill the other guy, 2. plow her after because that's what you'd need to do to "override" the first man's sperm and ensure the survival of your genes.
It sounds weird to talk about it this way but that's what it really is. Anybody familiar with history will tell you about new rulers killing pregnant women/babies of the last guy and shit because those kids will grow up to be competition. Back then, you didn't want women around you that weren't carrying your kid, or waiting to carry your kid. Sorry to make it so dark on a porn forum but i'd encourage you to look into the subject. It strikes the same instinctual chords as Harem which is "further the survival of your genes at all cost, even if you have to impregnate 20 women to do it." It's just a different strategy if you will.
TL;DR - It's normal to feel shame via the standards of today's society where monogamy is encouraged/rewarded, but dudes have been getting boners to NTR since caveman times.