If you are on Elena route, she is confused. The events made her unsure of the F's intentions and caused her a lot of stress... For what I have seen it is just a kiss. Plus If I am not being faithful (at least 2 other girls besides Elena and going after the photoshoot girl too) she can have her fun too.Answered like a true lawyer. LOL
But yes Elena can't be unfaithful if she is not in a relationship, and it is doubtful Elena could ever be in a long term monogamous relationship (even with the father or D).
I think she is willing to have F as her sole partner but she needs to figure out what F really feels about her.