OK, I'm gonna be straight:
No. of Pokemon with:
- Full chance of being male: 19
- 87.5% chance of being male: 101
70% chance of being male: 19
Full chance of being female: 24
70% chance of being female: 24
No gender (genderless): 75 (76 if Diancie is counted)
50% chances of either gender: 456
Yes, I've been counting. If you see, the number of species being more likely to be male is far higher than that of females. So for using Attract better, go for a Female gender, it's the safest one.
However safest can only go so far. You've got 139 odd Pokemon that prefer male nature, and 456 that havea 50% chance of being male, but remember that the rest of the whole of the Pokedex will stay unaffected by Attract. Also, since many Pokemon also have no gender, and some Pokemon also have the ability Oblivious, quite a bit of the game laughs at Attract.
Hope I helped!