No, there are two non-romanceable characters that Marion can have brief romantic encounters with (one in the prologue, one in Sherwood Common) before she gets involved with the romanceable characters. Doing so does not negatively affect your relationships in any way, but may change some dialog with the non-romanceable characters later on.
Once you pick a love interest at the end of the common route, you can't kiss anybody else *except* in Geoffrey and Gui's shared route, where the actual romance choice comes later on.
There aren't many choices in the game that negatively affect your romances. There are a few choices that let you specifically choose to end a romsnce (leading to the Sherwood friendship ending for Robin, John, or Will) and occasionally a bad ending choice will be relationship related. Otherwise, your choices lead to one of two good endings per route!