I think I disagree with some of the responses to this question. I do not think a spell would necessarily fail if it were cast by a male on females.
I could see a male wanting an only female response to a love spell that he cast. Its absurd to me to actually think that some one could not cast a generic spell towards just one sex or another. Especially since its common practice to have ones altar dedicated to both the male and female aspect deities. After all in Wicca we have The Sun and The Moon. We have symbolism for both female and male...mountains, birds, trees, water...black, white,...rocks and plants...
I do not understand why you would expect it to fail as long as it does not harm anyone. I think it would be worse if you sought a result from a spell in vain and fruitlessly.