So I understand you're parents don't consider you matured/strong enough to travel to another country on your own. I understand you don't mind losing out on a Green card, because if you can't go with a spouse, then you're not interested in going. This is typical in some cultures. I know when Gandhi left India for UK, his parents made sure he married someone before he left his homeland as a moral safeguard, to pre-empt his mind from getting temptations in a foreign land. However his wife stayed behind, but in touch. American parents also fears for their children when they go to another city for college, as they may do something young (foolish). I say you decide yourself, what's best for yourself. You seem strong to accept your fortune either way, in US or Nepal, so you will be happy to do as you want. I will say today's world is much smaller than in Gandhi's time. If you have problems far from friends and family, you're just an email/instant message away from advice you trust.
Perhaps you need to invest more in a good communication line with parents, which will withstand the tests of immigration. You're lucky technology has come a long way. Perhaps you don't need spouse (yet) to keep on the straight moral line, if you have instant advice via your mobile. You may check in regularly with parents for advise, a luxury people didn't have a few decades ago. Then you may find a good spouse to marry inside America also, you never know. There are good people in every country. If your parents wishes are so important, being relegated to staying in Nepal in case of a US no-go, sounds equally happy to you. Well it seems you have nothing to lose. Decide what makes you feel better. You can ask your parents if they'll accept you if you marry an American. It's not a question of country it seems, more a question of keeping people you trust and care about happy. I'm sure you know how to do that better than outsiders. It's a matter of your own choice. You're young and perhaps you want to develop a career before marriage. It depends on yourself, what you want. Nobody can tell you, since any risks of course, are always your own. Tell your parents about these risks, and what you think, and you will find your choices may be very simple.