Why can't men desire only one woman?

I read books often & I've noticed a characteristic that the male lead role has that I really wish real men would have. The male only wants the lead female character & has no desire for any other woman. She is the only one he wants & other women don't even tempt him. In real life, men are not like this & are checking other women out. They may choose to be faithful on the outside, but are cheating on this inside with lust & desire. I know it seems like an impossible thing to ask, but why can't men only want one woman?

Also, please don't go into some large explanation about evolution & how men can't help themselves because they need to spread their seed as much as they can. That argument just really irritates me because I don't think men are helpless animals that are victims to their own bodies.
+1 y
Everyone can stop answering this question, some people are being very rude, & I'm done with this. Thank you to the people who wrote nice answers & didn't act like a jerk.
Why can't men desire only one woman?
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