As a person in the interesting scenario of being in a long term relationship person that I lost my virginity to, I can say that it can strengthen a relationship. This could be unique in that exploring sexuality together (we were both virgins when we started dating) was something that only "we" will have, so that gives us something special.
What so many people seem to ignore about chastity-until-marriage is that, regardless of what you think, the first time, especially if its for both of you, will be awkward. Holding off in exploring what is a significant part of any healthy relationship until marriage is simply preventing the relationship from fully forming until it is too late to do anything about serious problems.
And anyway, personally, I see marriage as somewhat arbitrary. I'm not saying that marriage is stupid and that I'm going to have a girlfriend for my entire life, because it does have value, and pledging commitment isn't a bad thing, but the idea that it does something magical to the relationship and now you're allowed to do things you weren't allowed to do before (ignoring social rules) is simply silly.