I think that probably anybody that feels it is ok to be in a "serious" relationship, and have a emotional attraction with somebody else, probably feels that way because they either are, or have been in that situation already. It seems like a contradiction, how can one be in a Serious relationship, and feel things for another at the same time? It makes about as much sense as the term married bachelor, or kind of pregnant, pretty ugly, yadda yadda, whatever those kind of terminologies are called.
If it is a physical crush because one think he/she is hot, no biggie, if somebody is attractive, there is no denying it.