I'm no master of women psychology; however, I have spoken to many about what they desire. From what I gather, when they tell you this "lie", they actually believe those words. As far as why the lie exists to begin with, well it all boils down to saving face. Women have to keep up their social status, lest they fall victim to the double standard of being labeled a "slut", while a men who is true about his desires (and acts on them successfully) is called a "playboy" -- with obvious differences in the connotation. I won't say too much here, but message me, and I can go more in depth.

"If there any girls reading this or any of you who understand girls really well, let me ask straight out, why do girls lie about their true intentions/desires always?"

If you actually think women always lie, then why even bother asking us, since you think whatever reason we give will be a lie?

I mean... yes, as a woman, everything I say is a lie.

But if everything I say is a lie, then I am actually a man pretending to be a woman. But if I am a man, then what I say may be true, which means that I might actually be a woman.

Seriously, though, your comment is misogynistic.

If a woman desires sex with a good looking guy but marriage with a rich guy, it's in her best interest not to talk about the former, lest it be difficult to manage the latter.

It's not so much that girls lie; rather, it's well documented people in general don't understand themselves all that well. Self-reports of internal motivations are notoriously misleading. That's why research like this (or just common-sense observation of what people DO, rather than what they SAY) is useful.

"Women mating strategies" by Elizabeth Cashdan:

http://www.anthro.utah.edu/PDFs/ec_evolanth.pdf