That's the burden women bear. At least they used to before so-called "men" became such sorry-asses.
Women have always had to deal with men who take an interest when the interest is one-way. That situation is easy or hard to deal with depending on how well the guy takes a hint.
I work with students with autism, and in that situation, the guys never take a subtle hint. Everything is very literal. Tell him you're washing your hair that night and he'll ask about another night, when you're not washing your hair. In that case "I already have a boyfriend" will save a lot of time, truthful or not.