I love books that cross the category boundaries. Unfortunately some people do not, and adding a bit too much romance to your sci-fi novel, or too-much sci-fi to your romance novel will likely piss off some readers of the genre and kill the rating, which career writers are forced to maximize for sales. The genres are forced on the book at the publishing stage and, unless you are an established author, picking the wrong one can be a disaster. (Been there, done that!)