Yes, you can be attracted to several other people while in a relationship. You're human, of course. But it doesn't mean anything as long as you keep liking them from a distance. You can read the menu but you can't order, know what I mean?
... it depends on what you mean by "relationship".
You can date several people at the same time. In fact, I think people under 25 are (generally) too young to be in "serious" (monogamous) relationships because they don't have enough "life experience" to make that type of decision.