While I don't personally see anything wrong with it, I do think that it is a valid question that may need a little more in depth study.
The desire to mate is obviosuly part of a guppy's natural life. And, frankly, I think that main reason a lot of people have a fish tank in their house is to bring a little slice of nature into their homes. And, apart from a few exceptionally rare circumstances, there aren't any all single-sex groups of fish in nature.
I do wonder if not having the presence of females about would mean that the males hormones aren't the same. Again, it doesn't necessarily mean that it is bad, but I wonder if anyone has ever done a study on it. What differences exist between a guppy grown up in a mixed sex environment versus a guppy grown up in a single-sex environment.