3a. If there are less than 15 women then it means that whatever metric you used to pick "best" may have an unconscious or unexamined bias. Now you have to decide which do you consider more important, sticking with your "best" metric or trying to find and eliminate that bias?
4a. If sticking with your "best" metric is more important, then you are now deliberately excluding female candidates for reasons that are fundamentally unfair to them.
3a doesn't make sense because the male to female ratio in the industry is not 1:1.
Because 3a doesn't make sense, 4a doesn't make sense.
Part of the reason there weren't more women speakers in the first place is because of fundamentally unfair practices prior to their applying to speak. The system is already unfair. The question is, what do you as a forum organizer do about it?
1. Find the 30 best speakers that are available and can speak to a relevant/interesting topic to your event.
2. How many of them are men? How many are women?
3. If there are less than 15 women then it means that whatever metric you used to pick "best" didn't select a 1:1 ratio. Now you have to decide which do you consider more important, your "best" metric or 1:1 gender ratio?
4. If 1:1 gender ratio is more important than best than you have deliberately excluded male candidates for reasons that are fundamentally unfair to them.