why do men pretend to be something we're not?

Neither men nor women are meant to be monogamous for life. They are meant to be together for enough to time to raise a child until it's relatively independent, which is about 6 or 7 years. It sounds cynical, but it's true.

Just because "spreading your seed" was necessary thousands of years ago, doesn't mean it's a justification for your behavior today. There are a lot of primitive impulses still in humans, but we also have reason with which to think rationally and overcome them. For example, rape was a part of ancient human behavior, and it was beneficial to the species' survival, as it resulted in more babies. But modern humans don't rape (most of them) because they can reason that it is wrong, regardless of whether or not it could result in impregnating more women. We don't do things nowadays based our primitive impulses, or at least most of us try not to.

Also, I think you are wrong to buy into the traditional idea that men have higher sex drives. It's been proven that women's are just as high. I'm a woman, and I totally understand the fear of monogamy.

Indeed. Societal expectations and laws completely butt heads with evolution and instinct. This will not improve. If our society continues to evolve in this lopsided way, both men and women will become even more oppressed in their own minds. And sadly, most will never even understand their own frustrations.

It's not about sex, it's about commitment and love. If you marry someone, you are committing to them because (hopefully) you love them. There's even some compelling research that women are also hard-wired for polygamy. Given that, marriage is an even more sacred thing, because it is a sacred union between two people, whose love for each other is strong enough to overcome biology and our hard-wired, baser needs.