I don't think it's bad. It's part of the human condition - looking at others, even if you're already in a relationship. In my opinion -- It's only bad if you've got a monogamous relationship, where you decide to cheat on your SO with this cute guy from work, or your boyfriend gets super jealous of your attentions towards this other guy, and you don't talk it out.

But if it's harmless fantasies, I don't see it being an issue. But that's me.