YC isn't about building romantic relationships, it's about building companies. Mixing business and romance is a losing proposition for women, because the latter taints absolutely everything. One of the awesome things about being a guy is that I can look around at, e.g., my classmates from grad school, and be quite sure that their "mental file" of me has a lot more about my intellect and character than about my looks. If someone has my back in an argument about what our team should do, I can comfortably assume it's because they respect my position, not because they're romantically interested in me. These are things men can take for granted that women cannot. And when you allow romantic advances in professional contexts, you make it all that much harder.