If you have a decent relationship with your parents and you respect them don't lie to them. Your BF is definately right about not hiding, it's sullies the relationship and would be generally unhealthy.
The rent is irrelevant. If you can't afford it you can find a roommate besides him. Drop it from consideration.
I'm neither for or against cohabitation. It's a relationship that should probably be approached in exactly the same manner as marriage. I expound at length on my opinions about pre-marriage planning in
another post so I won't repeat it here. I would suggest that you either take the relationship seriously or don't live together.
Your ages are irrelevant except that I think you're too young to get married. <img border="0" title="" alt="[Wink]" src="images/icons/wink.gif" /> By all means be together, enjoy being together, but I really wouldn't suggest tieing yourself down just yet. If it's really right it will still be right in a few years.
It sounds like you want to live together and the issue is your parents. You won't like this (and I don't blame you) but if they're going to sign the lease you should talk to them. Point out how respectful you're being by asking their opinion and mention that he didn't want to hide things from them either. Best case is that they respect you and trust that they've done a good job raising you and let you make your own decision, probably after they express their opinion. Worst case is that they dis you by saying no in a very emphatic way.
If they say no don't knock them too much for it. Parents often think they know best, sometimes they're even right. They also want to protect you and often that interferes with their ability to see you as an independent person.
Either way
you still have to make the decision. At least now you know where they stand.
Sorry I can't decide for you but seeing as you're smart enough to look for advice I'll trust that you're a big girl and capable of figuring things out for yourself.
BTW, sorry but I have to say this: Use birth control, at least two kinds, every time, no exceptions. Children can be wonderful but they can also be very stressful, especially if you weren't planning on having them just yet.