That sounds a bit silly. There are always going to be those total b******s in society that just want to use people, but that's by far the minority. In my opinion (and I hate to stereotype, which is exactly what it seems like I'm about to do), it seems that these people are attempting to scare you into not having sex for their (or your) religious beliefs.
I'm not saying that this is a good or a bad thing, but it just seems that way to me. (I don't want to be slated for expressing my views.)
If you're in a loving relationship, 'respect' should and in all likelihood always will stay there through sex, and if not, then you shouldn't be with him anyway - the other option is waiting until you get married before having sex, then him losing respect for you. I'm sure the first option is more preferable to the second.
If a boy's going to lose respect for you after having sex (I'm not sure why he'd lose respect... but anyway), then he's going to lose respect whether you've been going out for a week, a month, or a year, and whether you're just friends, are going out, or are married.