For starters, I will grant you that many couples go longer than 1.5 years before getting engaged. What is the boyfriends rush?
Why would he? Are you really worried about you getting sick of him? Married couples that go the distance don't get sick of each other. They're fine wine - better with age.
Don't. Pay attention to it. "Water the grass" so to speak. Keep sex interesting.
What if you ask to many what if's? Seriously. It's possible that you could end up taking each other for granted. It's also possible that you'll be best friends that truly cherish each other.
"Forever" makes me laugh given that your lifespan will be infinity orders of magnitude shorter than that. But a 50-60 year marriage is possible. By that time you'll be in diapers, wheelchairs, and a retirement community.
What if you ask more what-if questions that you'll never be able to answer? What if you get cancer? What if you have an aneurism? What if you get hit by a bus? What if you get taken by a serial killer? Fuck - may as well load up the pistol and put yourself out of the misery of what if.
You don't. If he does, you survive. You live. You continue. You eventually die. You go in the ground and decompose. Ice ages come and go. Asteroids strike and mass extinctions come and go. Eventually in two or so billion years the sun will expand many times greater than its current size and become hotter, burning of the earth's oceans and eventually the earth will either be absorbed or loosed from its current orbit. Your life is - no offense - completely insignificant in the grand scheme. You are the only one that gets to enjoy your life - no one else will, no one else can. You may as well take chances. Once you're dead you can't take any chances. Just take smart chances. Marriage is a smart chance if you don't marry a jerk.
Yes. Who decides? He does. I could marry a prostitute if I wanted to - some men have. What makes someone "good enough"? It's the connection, the friendship, the companionship. Reality isn't romeo and juliet, it's dick and jane.
What if again. Is he a total loser? People learn to handle people. My friend and I were actually talking about this yesterday - dealing with our spouse's bad sides. Here's a suggestion: why don't you find out? What's the worst that could happen? Divorce? You might be surprised to learn that people that divorce can and actually often do go on to lead happy lives.
All I will say is that it's OK to give it some more time. Ask him what the rush is? Why not wait for a 3-year anniversary before getting engaged? But at some point you gotta shit or get off the pot.