There is absolutely no clear line; it's all up for interpretation, both by the individual and by the culture itself.
To my nose, two rose fragrances, Iquitos ( ostensibly male ) and Gucci No. 3 ( ostensibly female ) both have almost identical bases.
In general, I'd say when people aim a rose at men they put either spices, incense, woods, and/or citrus into the blend to make it more abstract and less of a literal rose, while on the other hand, you're more likely to find powdery or gourmand roses aimed at women. Pure roses, too, are more likely to be aimed at women. However, all these are very wide generalizations, and I can think of many exceptions to both.