I understand and appreciate your view. I feel it is important to establish if the picnic was an official event, held on a (normally) working day, and therefore, mandatory. In which case, normal rules would apply. Hence, the need to establish the reason for absence, particularly if the company is spending resources for employee engagement and motivation.
If it was NOT an official event, then attendance should be discretionary (but desirable), in which case no action should be taken because the picnic becomes a purely social event. I agree with you where a group (in your case, all female staff) is uncomfortable, then the WHOLE group should be excused (although this may actually defeat the purpose of a picnic if it was for employee engagement/motivation purposes).
On a more humorous note: my children love chocolate so much, I fear I would never have the opportunity to see if I can FORCE them to eat one!!!
Regards, Harsh