I am 19 next month, so by all means disregard my opinion as I am not a parent and am biased as I am in this situation myself, but here is the child's perspective:
I am 10 months into a relationship with a guy who I have known for 5 years and is a month older than me. This is my second serious relationship; I was with somebody else from 14 until 17, and I first slept with him when I was 16.
I understand that my parents did not want me sharing a room with my first boyfriend, but I do think I should be allowed to share with my current boyfriend; we are both adults and sensible and responsible etc, and rest assured, the last thing I want is my parents hearing me having sex! I am on my gap year and he is at a 6th form college, so over the next year we will not see all that much of each other so being able to have this extra closeness would be nice when we do see each other.
There is also an element of peer pressure. Do ask what friends are allowed to do, because it might have prompted the idea. In my case, I have a friend and his girlfriend who I and my boyfriend socialise a lot with, double dating etc. Both their parents allow them to share a room whenever they stay over. Obviously my lovely friends do not ridicule me for not being able to! But I am hugely jealous of them, and I do get comments from them like "That's such a weird rule" and "Why are you still ordered about like a kid?" and its...not fun