Uh... I suspect ~LeOtherGuy is responding that way because you seem to have jumped to a conclusion about here that's not in the show or this comic?
Unless you're reading her assuming that the flowers are for a dragon as some sort of disapproval of him possibly liking a pony? I can see that reading, but it's not the first one I would have jumped to. That'd be kind of like saying that her assuming it was a "girl" dragon at first was anti-gay... I'll grant that it's awkward and could be a bit insensitive, but it doesn't mean she has any sorts of views on male-male relationships beyond maybe thinking they're in the minority of relationships (and therefore not the first thing to guess). :/
A female dragon is a far-fetched assumption, considering there aren't any dragons living among ponies besides Spike. A mare or filly should have been Roseluck's first guess because it's the most likely. Her second guess being a colt means that Spike being gay is easier for her to accept than a dragon wooing a pony.
Spike spent an entire episode off hanging out with other dragons, and almost none of the ponies know what happened during that time. I see what you're saying, but I'd still interpret this as her honestly mis-guessing rather than having some sort of deep-seeded prejudice.